It starts out kind of by accident.
Tony just went to go make some popcorn for their movie marathon (he and Bruce are still trying to get Steve caught up on all the great films he missed, something SHIELD seems to have neglected), and when he comes back, his boyfriends are kissing on the couch. Tony whines indignantly at having been left out of the action, prompting Bruce to teasingly flip him the bird and Steve chuckles against his lips. They separate just as tony is settling down with the popcorn to watch.
"Don't stop on my behalf!" He says quickly, putting the bowl aside and shuffling on his knees to kneel on the carpet in front of them both.
"We never do," Steve assures him, earning a swat to his leg, but he's smiling that signature Captain America smile and looking so damn sexy-adorable that both of his boyfriends lean in to kiss him at once.
The result is a surprising, chaste smush of lips, each of them catching some of the others' mouths against their own. They pull back and send questioning glances at each other.
"Is that a normal type of kiss now?" Steve asks skeptically at the same time Bruce says, "Well, that was interesting," simultaneous with Tony's "How have we not done that before?"
They all laugh a bit nervously at the overlap of voices and the uncertainty of the situation. Only a brief moment of silence follows before they all lunge forward at once.
Their tongues clash rather messily in the middle, a strange new mixture of familiar tastes as they slide together, trying to get into a rhythm. It's like trying to dance with an extra person, and like everything else in their slightly unconventional relationship, they quickly figure it out.
There's so much moaning and sighing they don't really know who's doing what, but it's such a blissful experience none of them really care. Tony's got a hand behind Steve's head and the other cupping Bruce's chin; Steve and Bruce's hands are entwined between them; Steve's other hand rests at Tony's waist and Bruce's strong fingers grip Tony's thigh. Their mingling breath is hot and they try not to move their heads for fear of knocking against each other, growing lightheaded and aroused. With all the new sensations, it's so much more intense than two-person kissing. Tongues take turns delving into different mouths, getting nudged and sucked in unexpected directions that send Steve's mind reeling and make Bruce's fingers tighten on Tony's leg.
Steve is the first one to reluctantly break away, so dizzy he thinks he's going to pass out, and his boyfriends' lips only linger together for a moment longer. They all stare at each other in quiet wonder, mouths hanging open.
"Well that was fucking hot," tony announces loudly, grabbing the popcorn and flipping down on the couch, legs across Bruce and Steve's laps. "You better save that footage to my favorites file, JARVIS - I think we just invented a new kind of porn. Title the video 'Mouth Threesome'."
"'Mouth Threesome' is saved, sir," the computer replies from hidden speakers, sounding a bit irritated at having his advanced technology used for crude entertainment.
"Thank you," Tony says with a wide smile, tossing a piece of popcorn into his mouth and ruffling Bruce's hair when he grins bashfully.
Bruce rests his head on Steve's broad shoulder and Tony starts Star Wars: Episode V, only about two minutes passing before the scientists start arguing about whether the fifth or sixth episodes were the best. Steve watches them fondly, rubbing Tony's feet.
He knows what they have is weird. He also knows he doesn't give a damn.