I've been looking for this fic for a while now: Bruce is living in the Tower with Tony and Pepper. Tony and Pepper are in a relationship but Tony is attracted to Bruce. Pepper notices. One day Bruce asks Tony if he can see the arc reactor and Tony takes it out of his chest and lets Bruce look at it. Pepper watches their interaction and Bruce and Tony are so intimate with each other that Pepper knows she has lost Tony to Bruce. Not a 3some!