It isn't that Tony is terrible at sharing - because he is, have you met Tony Stark recently?
It's that Bruce is just as terrible.
Pepper doesn't really mind, though. She's grown accustomed to being the rope in a really adult version of tug-of-war; whether it's Tony's grabby hands on her waist or Bruce's insistent pulling on her hair. It gets even better when all three of them are naked and Tony and Bruce decide to get competitive.
Tony's decided on some ridiculous set of rules - they can't use their hands, and she's blindfolded by one of Tony's ties - and she stretches back against the mountain of pillows ("why do you need fifteen throw pillows, Tony?" "Bruce, Bruce, Bruce, you never know when you'll need an extra one"). The silk is cool across her eyes, and she bats Tony's hands away from "checking" its tightness. Bruce laughs, somewhere to her left, which means it has to be his mouth brushing lightly over her shoulder and trailing up her jaw.
Pepper tries to reach out, wanting more of a clue as to who is where, but Tony makes a disapproving sound and his hands come from somewhere to wrap around her wrists. He presses a kiss to her left temple - he could be anywhere, this is frustrating beyond all measure and Tony knows it - and then to her mouth. Then the brush of goatee stops, and Pepper groans before another mouth replaces it.
Bruce and Tony trade off kisses: slow and teasing, flickers of tongue and denying her more (that's Tony); hard and dirty, almost too rough (Bruce, of course). Each one refusing to let the other have more than a few seconds, and Pepper almost can't breathe.
"I'm going to-"
"I don't think she's in much of a position to do anything," Bruce remarks offhandedly. "Don't you, Mr. Stark?"
"That was the point, Dr. Banner, was it not?" Tony asks, innocence in his voice. She can just picture his smirk, though.
"I thought the point was to make her come with our hands tied behind our backs?"
Tony's laugh comes from right below her breast, warm breath raising goosebumps on her skin.