Here be m/f/m and some light m/m/f.

Part 4 - New Player

John fidgeted in the seat of the cab as it drew closer to Baker Street. Sherlock's text had been unusually cryptic, even for him: Come home, there have been developments. Need you.

No matter how many times John texted him back, he'd received no other response, and he'd known better than to try and actually call his flatmate. His frustrating, annoying, incredibly exciting (in oh so many ways!) flatmate. He'd known how long John had been hoping for a date with Mycroft's PA, Anthea, and to interrupt him just when things were getting interesting…frustrating. Very, very frustrating. But…developments. What did that mean, exactly? And the last bit – 'need you' – not very Sherlocky. Needed him for what?

A case, most likely, although John's libido had its own hopes; he might have been seeing only women for the past ten years of his life, but wasn't shy about admitting he was bi-sexual (well, except the time he'd told Irene Adler he wasn't gay, but he had no regrets about lying to her). But Sherlock was the most amazing Alpha – hell, the most amazing human being, period – that John had ever met, and his icy self-control was nothing short of astonishing. Even after two years of sharing a flat – and another two years mourning his supposed death – John still had no idea if Sherlock was truly asexual (a supposed biological impossibility), or simply repressed his normal urges so well that he might as well have been. And if he had a preference for either men or women – or both – well, that was as much a mystery as who killed Jack the Ripper.

The cab came to a stop, and John paid the driver before retrieving his overnight bag and heading into the building. As he started up the stairs, his nose twitched at an unfamiliar smell; it took him until he'd reached the upstairs hallway before he identified it.

Omega. An in-heat Omega at that. What the hell? He pushed open the door to the flat cautiously, leaving his overnight bag sitting in the hall, not sure what he was about to see – well, that wasn't entirely true; of course he suspected what he was about to see, what else could it be? His cock twitched in his trousers in response to that enticing smell, but he did his best to tamp down on his body's reactions, at least until he had a better understanding of the situation.

As he stepped into the sitting room, his eyes confirmed what his sense of smell had already told him. He gaped as he took in the sight before his eyes; not only was there a woman sleeping on the sofa – a naked, sheet-covered woman – but it was one he recognized. "Molly?" he said disbelievingly. "Molly Hooper?" No, it couldn't be! Molly was a Beta – a weak Beta, but still a Beta. So how the fuck – ?!

"You know why she did it, John." He jumped a bit at that familiar voice – and his earthy,tantalizing scent – coming from right behind him. Sherlock. And Molly. Of course; if Sherlock was going to get romantically – or at least sexually – involved with anyone outside of Irene Adler, it would be Molly Hooper. But if Molly was an Omega, an unregistered, undeclared Omega who'd been illegally hiding her status, then the only reason she'd have come out to them... "My brother's doing," Sherlock said, and John felt him lay a hand on his shoulder.

John felt the heat of that touch through the layers of clothing he wore, and shivered. Alphas didn't typically share, but the Omega Plague had changed a lot of things in their society. With only one Omega for every hundred Alphas, well, they'd had to adapt. Just because their biology retained a great many primitive aspects didn't mean they were incapable of evolving.

"She's ready for you, John," Sherlock murmured, his voice very close to John's ear, a low rumble that affected the other man almost as much as Molly's in-heat aroma. She murmured and turned onto her back, the light sheet that had been covering her from the waist down falling to the floor, and John heard a bit of a growl escape his lips before he could stop it. "We've already knotted, she and I, and she's pregnant so my dear brother can have no complaints that I called you home early. Now it's your turn...but there's just one thing." The hand on his shoulder turned to a tight grip, and John managed to tear his eyes away from Molly's sleeping form long enough to give Sherlock an inquiring look. His flatmate leaned closer, so that his mouth was almost close enough to touch John's as he said: "I want to watch."

Breathing hard, John did the bravest thing he'd ever managed in his life; he leaned forward and pressed his lips to Sherlock's, placing one hand over his friend's and holding it on his shoulder, while the other reached up and grasped Sherlock's dark curls in a firm grip. "Maybe I don't want you to just watch," he growled when the kiss ended – his decision, he noted with a distant part of his mind. Not Sherlock's; his. The other man had allowed the kiss, accepted it, even – and returned it just as John broke it off.

"Molly!"

She stirred at Sherlock's terse bark, blinked and opened her eyes and stared, open-mouthed at the sight of the two men standing in so intimate a pose. Then she licked her lips and swallowed, and John knew that, whatever happened next, Molly would be as much a willing participant as the two Alphas she'd been given to. She didn't say anything, however, just rose obediently to her feet and walked over to where they stood in the flat's doorway, completely unselfconscious in a way John had never seen her act before.

If his dick hadn't already been hard as the proverbial rock, that alone would have ensured an erection tenting his trousers. His eyes darted down, his tongue coming out to taste the corner of his mouth as he saw that Sherlock was just as powerfully affected by Molly's nudity – or was it by John's kiss, John's hand still covering his own, the fingers of his other hand still clutching Sherlock's dark curls in his fist?

Both, he decided as he watched Sherlock watching Molly, saw his friend's eyes darken as the pupils expanded and nearly swallowed up the blue-green of the irises. Just like John, Sherlock wanted both the Omega and the other Alpha in the room. And as much as John wanted to blame Molly's heat for their mutual arousal, he was clear-headed enough to acknowledge that the attraction between the three of them was nothing new. He'd always thought Molly was cute, but her unwavering admiration for Sherlock had been as obvious as her love of bright colors and morbid jokes, and so John had never bothered asking her out, recognizing a lost cause (so he believed) when he saw one.

Now, however, Molly was his for the taking – and so was Sherlock, his attraction to whom John had fought even harder once the consulting detective made it clear that any sort of romantic or sexual entanglement was off the table, on the very first night of their friendship.

John kissed him again, even as he reached out to pull Molly closer, her scent mingling with Sherlock's in a heady, enticing aroma like no other. John's own scent was getting stronger, even if he was so used to it he barely noticed it anymore; he saw Sherlock and Molly's nostrils flaring as they inhaled deeply, and joined them in a ritual older than written history. Eyes closed, faces turned toward the ceiling, just breathing in each other's presence. The spell was broken when John felt a pair of hands tugging impatiently at his jacket; he opened his eyes and saw Molly reaching for the snap to his jeans, and suddenly there was a flurry of motion that ended with him as naked as the other two, Molly on her knees with his cock in her mouth and Sherlock kissing him while one hand buried itself in Molly's hair and the other slid down John's back to fondle his ass.

The urge to knot was overwhelming, overriding the pleasure he felt from Molly's mouth and hands on his cock. He broke from the kiss, dropping to his knees and kissing Molly instead, his tongue sliding against hers, the warmth of her body and the ripe scent of Omega heat filling his senses. Soon she was on her back, legs spread wide as he positioned himself over her, while Sherlock reclined next to them, sharing kisses with Molly and nuzzling at her unmarked throat. John growled a bit at the sight, but then Molly smiled and pulled him down so he could nuzzle the other side, and she lifted her hips and gave an inviting whine as he pushed his way into her and Sherlock placed a hand on his hip, skimming his fingers down the cleft of his ass and John exploded into movement.

He prided himself on being a caring and attentive lover, but it had been a literal decade since he'd knotted an in-heat Omega and he'd forgotten how entirely overwhelming it could be. His Alpha nature took over, and he found himself nipping at Molly's throat – but not hard enough to draw blood, he managed enough control for that, thank God – and thrusting into her at a furious pace, too absorbed by his own need to reach completion to worry about being too rough.

Thankfully, as he would later discover, Molly was perfectly fine with that. The sweet little pathologist would turn out to have a definite taste for the rough side of sex, which would stand her in good stead over the next few days. Nor did he need to worry about her reaching fulfillment; she was an Omega being knotted by an Alpha and had no difficulty orgasming. Especially once Sherlock reached between their fiercely rutting bodies and slid his finger along her clit while at the same time alternating kisses with her and John.

He felt his knot rising and gasped out a strangled warning. Sherlock pulled his hand out and returned his attention to Molly, kissing her hard while John once again buried his nose in her neck, his teeth and lips on her clavicle. She cried out as another orgasm washed over her, and that did it; John's movements became shallow until with a hoarse cry of his own his knot formed, locking them together while his cum continued to spill inside her.