Steve and Bucky both have two soulmates. When Bucky falls, Steve goes into the ice not long after. He wakes up decades in the future and has a rough introduction to his other soulmate, Tony Stark.
Neither man seem too enthused about the revelation, and Steve is slightly surprised when Tony makes no demands of him or even uncovers his soulwords. Turns out, Stark doesn't think too highly of soulbonds. Still, they manage to work together and Steve comes to realize he might have misjudged Tony.
He's working up the nerve to try and make this more than a friendship when Bucky turns out to be alive. And not quite alone.
Or, Steve is jealous of cuddles and bed sharing, Tony has no expectations of his soulmates because his soulwords are extra crappy, Bucky is sharing his psyche with the Asset, and all of them are on a converging path to falling in love.
Ultron is destroyed, the Avengers are in disarray, and the Winter Soldier is still in the wind. Steve knows that he has to fix the ever-growing ocean of distrust between Tony and himself, so he takes a leap of faith and tasks Tony with the most important thing: finding Bucky Barnes. But it takes a pair of sexy but stolen hand warmers, several robots, Hawkeye and countless selfies before Steve realizes the immensity of what he's set into motion.
Tony Stark receives an invitation to Steve and Bucky's wedding. He has absolutely no idea who they are. For their chutzpah, he decides to pay for everything for their wedding- and hop into the planning to make sure they get the best of the best.