This is set in season four, episode 19, "Pictures of You" where Damon and Stefan try to provoke Elena into turning her humanity back on. It starts with the scene of Stefan and Elena dancing and goes AU from there. Rated M for a reason and evolves into a threeway.
Stefan challenges Elena "So you're just here to help Rebecca get the cure, right? On what happens to be the most sentimental night in high school?"
"You think this is my cry for help? I'd be happy to show you what a real cry for help sounds like" Elena replies, undaunted.
"So, this, us. You feel nothing?" Stefan asks.
"I feel nothing" she replies.
"I don't believe you" he says.
"I don't care" she replies.
"So you don't remember what it would feel like when we would dance? When my hand would touch your waist?" he asks.
"No" she stubbornly replied.
"How 'bout this. When our fingers would touch?"
"Nothing."
"And this? Does your heart really refuse to remember?" What heart?
"Oh, I remember more than what it felt like to dance together. Do you remember what it felt like to do this?" Elena held eye contact with Stefan as she pushed up on her tiptoes and slowly pressed her lips to his and then deepened the kiss. He started and pulled back trying to figure out if he was getting through to her or if the plan to turn her emotions on was just triggering his feeling for her instead. He had meant what he told Damon about bowing out and starting fresh after she turned her humanity back on but he couldn't do this and keep up his end of the bargain. She watched the struggle wash over his face and slowly shifted her gaze over to Damon, who had been watching exchange while glowering at them.
She looked between the brothers, knowing that Damon could hear her clearly, even across the room. "I know what you boys are up to. And I'm guessing you even have an idea who 'gets' me if I were to turn my humanity back on. But don't you understand that was part of the reason I had to turn it off in the first place?" At this the brothers exchanged a guilty glance. "Elena, let's all go outside to talk about this. This isn't the right time or place".
After checking on Rebecca dancing with Matt she decided she could chance a few minutes away while the boys tied themselves up in knots explaining how they planned her future without her. This could be fun. She strode outside past the slideshow and found them standing together, Stefan running his fingers through his hero hair trying to hide his stress, and Damon, faking calm, cool and collected while looking at a picture of her and Stefan dancing together at the first decade dance.
"So what is your master plan for me, masters?" she asked sarcastically, bobbing her head like Jeanie. "Our plan" Damon smirked, "is for you to turn your humanity back on and stop being such a giant bitch. You keep it up like this and you're gonna get caught by the council and staked. Not to mention, you are alienating all of your friends. Nobody keeps their humanity off forever and the longer you go on like this the more you're going to have to deal with when you finally do turn it all back on."
"Blah, blah, blah. I notice you didn't answer the 'who gets to keep the girl' question. I'm all ears" she looked between them, expectantly.
Stefan hesitantly started "Elena, I've realized that I'm repeating the same mistakes over and over, just like Katherine. If you turn your humanity back on, I'm going to start fresh, create a new life for myself"
She cut him off "so, that's it? You don't feel anything for me but were just trying to manipulate me into turning my feelings back on and then were going to ride off into the sunset? That is notwhat it looked like while we were dancing and we both know you're not that good of an actor. The sad thing is, I might have been able to deal with Jeremy's death with both of you by my side. But I couldn't deal with that and the loss of one of you. So, I have my own master plan. I'll offer you boys a deal. Either share me and I'll consider turning my humanity back on. Or you both lose me and my humanity stays off and I will be the one who rides off into the sunset. I'm sick of Mystic Falls anyway".
Damon and Stefan shared a look of alarm, trying to figure out how to respond. "Elena" they both said at the same time.
"I've got to get back to the dance. You can give me your answer tomorrow morning. Ciao!"
Both boys' shoulders slouched and they got busy looking at their shoes as they considered her speech.
"Well, well, well, and it is back to 1864 for you two all over again!"
Both of their heads shot up as they saw Klaus cockily approaching. "Klaus." Stefan muttered. "I suppose we have you and your sister to thank for this."
"Me? Oh, I think she was well on the road to wanting both of you before I took you on our little tear along the Eastern seaboard. She was just too guilt-ridden to voice it at the time. Do you really think you two would have lived happily ever after if I had let you stay? She was obviously already attracted to Damon and that was growing. And did you want to watch her grow old and die? You would have only looked appropriate as a couple for a few more years anyway. Can you see her being happy having to move towns every few years, never letting anyone know her big secret? Rebecca and I did you a favor. Elena could be a glorious vampire. Besides, sharing a doppelganger with your brother can be good fun as you well know. It couldn't have been all misery with Katherine."
Stefan growled "get away from here, Klaus!"
"Silas, actually", he smirked.
The brothers' jaws both dropped.
"Too easy to get inside your heads, boys. Ciao!" and he disappeared.
"Well, this just gets better by the minute, doesn't it?" Stefan gritted out.
"We better get back to the dance and make sure he doesn't turn the prom into Carrie", Damon replied.
As the brothers walked back into the boarding house after saving Elena from Bonnie, Damon walked straight to the bar and poured a glass of bourbon, cocking at it Stefan in question. Stefan nodded. Damon poured a glass for himself and they both sat down on the couches facing each other, neither having the slightest idea where to begin, almost too tired to talk.
Damon held his head in his hands, considering the situation. Slowly, an idea came to him.
"Stefan, I think I have a plan, but I don't think you're going to like it. I don't even like it, but it might be our only hope."
"What?"
"Well, what if we 'share' her until her humanity is stable, and then we can go back to the original plan?" Damon proposed.
Stefan stared at him and then dropped his gaze again.
"What do you mean, 'share'?"
"Stefan, what do you think I mean? Do you think I like this idea? Just seeing you dancing with Elena, kissing her tonight killed me. But if we are going to get her to turn her humanity on, I think we'll have to do it. Besides, that psychic freak is out there and we can't be dealing with him and Elena like this at the same time. This is distracting us and making us easy prey. You saw how easily he got in our heads with his little speech as Klaus!"
"Damon, I don't know if I could leave if I am with her again. It was hard enough faking the boyfriend act tonight. I don't know if I'm strong enough."
"You think once she turns her humanity on she will even want us both? Don't let Silas get to you. Elena is not Katherine. She is too much of a goody-goody at heart for this. I think we need to at least try it at see what happens. We'll get through this. After all is said and done, you are my brother."
Stefan looked skeptical, but shrugged. "Ok. I'll go along, but I don't have a good feeling about this."
The next morning, Elena strolled into the boarding house living room as Stefan was writing in his journal and Damon was drinking blood and bourbon mixed in a tumbler staring at the fire. "So, what's the verdict?"
Stefan put away his journal and stalked over to her, as Damon did the same. Damon slipped his arm around her waist as Stefan cupped her face with one hand, gazing into her eyes with amusement. Damon put his hand on her other cheek, bringing her eyes to his. "We'll be good brothers and share if that's what you need, Elena. But you have to turn your humanity back on."
She tried to hold her composure, or at least fake it, but she was already blushing.
None of them had noticed that Katherine had walked in to witness this scene. "Well, isn't this hot. I told you it was okay to love them both. I was planning on a visit now that Klaus is out of my hair. But I think I'll leave you to it!"
"Why is Klaus 'out of your hair'?" Damon vampsped over to her before she could leave. "Oh, he had some very important business to attend to with a witch named Sophie Ann Deveraux in New Orleans. We are all safe from him for now. But I'd hate to interrupt this, so I'll find my entertainment elsewhere. Have fun, kids!" Katherine used her to her advantage and twisted out of Damon's grasp and out the door in a flash.
Elena cleared her throat. "Ok, Klaus is gone. Katherine is gone. Now, where were we again?" She smirked. Then she turned and walked slowly up the stairs. "You boys coming?"
Damon smirked at her. "That is definitely a girl who is asking to be chased, brother." He was pretty sure the fallout from this was going to be terrible, but was determined to go through with this and he knew that if they didn't at least try this now he never would. He started up the stairs after her and Stefan somewhat reluctantly followed. He had a feeling this would end terribly in the long run, but couldn't deny there was a part of him that secretly was looking forward to being with Elena again very much, even despite his brother's presence.
Elena was very surprised the brothers had decided to take her up on her offer. She had been fully expecting to hop in one of their cars and ride off into the sunset alone. Once she reached the top of the stairs she quickly decided Damon's room would be best since he had the largest bed. She quickly stripped down to her black lace bra and panties and kneeled on the middle of Damon's bed, waiting for them to come for her.
Damon got there first and froze at the doorway at the sight of her, eyes gleaming and daring them to go through with this. Stefan soon came up and hesitated beside Damon before walking in the room as well.
"Well, what are you waiting for? Don't tell me you're getting cold feet now." Elena demanded.
Stefan decided if he was going to do this he was really going to do it. "Come on, brother, we've got a date with our girlfriend." Stefan slowly walked to the bed and climbed on behind Elena, still kneeling there watching Damon expectantly. He kneeled behind her, legs staggered with hers. He wrapped his right arm around her waist and pulled her to his chest and grabbed her left arm with his, pulling her wrist behind his head. He looked up at Damon, still frozen by the door.
Damon shook himself mentally and walked to the bed, climbing on in front of Elena, holding eye contact the whole way. He knelt in front of her, her chest flush with his and took his right arm and hooked it around her back, forming a tight circle around her with Stefan's arm. He took his left hand and pulled her neck forward to kiss her, slowly parting her lips and licking her fangs as they dropped.
"I love you so much, Elena. Come back to us. Be here with us." He pulled back and saw her eyes were glistening. They were finally getting through to her and he could tell she wouldn't be able to get through this without her humanity coming back no matter how hard she tried to keep it off. This strengthened his resolve and he finally stopped worrying about what the repercussions might be.
He caught Stefan's eye and nodded slightly, turning her to face his brother. Stefan leaned forward and started kissing her deeply, as Damon took his shirt off, then unhooked her bra. Damon then took of his jeans, leaving him naked and Stefan slowly turned Elena around, placing her in a straddle over Damon. Elena leaned down and started grinding on Damon and making out with him as Stefan took his clothes off, watching them intently the whole time. Then he reached down and slid Elena's panties off as she made out with his brother. He climbed up and straddled Damon's legs behind Elena, pressing his dick into her lower back and massaging her breasts.
Damon sat up and started kissing her and his fingers slowly trailed down her stomach and circled around her core lightly, getting two fingers wet, finally making contact with her clit. He kept eye contact with her as he fingered her and kissed her.
Stefan laid her on her back on the bed upside down from him, and replaced Damon's fingers with his mouth, relishing her wetness and lapping her up lazily as Damon continued to make out with her and started massaging her breasts and pinching her nipples. Stefan's breath caught in his throat as Elena's hand grabbed his cock and heard Damon's a moment later as he glanced up and saw her take his brother's as well. Elena's breathing picked up as Stefan began licking faster and harder, then pushed two fingers in, pumping too slowly to let her come just yet.
Damon decided he wanted to taste her and pulled Stefan back by his hair and gestured. The brothers had done this long ago with Katherine and were both a bit disconcerted at how naturally this was coming back to them. Stefan knew what Damon wanted and pulled back and they repositioned Elena so that her legs were hanging off of the bed and Stefan straddled her shoulders on top, sliding his cock into her mouth. She sucked him in voraciously and he wasn't sure how much longer he could last. Damon knelt on the floor in front of her and she came hard just moments after his mouth enveloped her core, licking inside, then sucking her clit into his mouth between his blunt teeth and pushing his fingers into her. He then stood in front of her and grabbed her hips, pushing in in one stroke. He pulled his fingers out of her and slowly started to push them into her ass. She almost gagged on Stefan's cock, but managed to keep going after a moment.
Damon pumped into her and was getting close, but wanted to try something else. He grabbed his brother's shoulder, cocking his head at him when he saw him. Stefan raised his eyebrows, a bit surprised but willing and he laid down on the bed. Damon pulled out of Elena slowly and they quickly picked her up and impaled her on Stefan. He lost no time in pushing behind her and slipping his dick, still wet from her into her cunt into her ass. She made a strangled sound, absolutely shocked as she had never had anything there before.
Stefan grabbed her hips and set them rhythm for them all slowly getting faster. "Getting close", he gasped out. Damon couldn't hold out much longer either, and asked "Elena?" She clenched in answer and Stefan lost his steady rhythm, pistoning in and out spastically, coming hard. As he did, she quivered all over and Damon pushed in twice hard before shooting a hot stream of come into her ass.
They collapsed, totally spent, Stefan to one side, Damon to the other. Both brothers looked over her at each other as she stared bright eyed to the ceiling, sighing in contentment for a moment. They were despairing for her humanity, but soon her eyes started to water and she pulled each of their necks close to her.
She unexpected started sobbing, "Jeremy is really gone, he is really gone!" and they knew they had gotten through to her. Putting aside the weirdness of being there with her naked and after this intense experience, they both started petting her and murmuring soothing things. "We're here for you 'Lena, we aren't leaving". They wondered at the sudden change and eventually she cried herself out and fell asleep between them.