Steve Rogers is not a typical Dom and Bucky is a Switch choosing to submit for his Captain. Together they have a love that breaks rules and flaunts expectations, and when they see a beautiful sub being mistreated by his Dom, Bucky and Steve break another rule and interfere.
Tony is a sub bound to Old World protocol-- seen but not heard, there for Ty's pleasure alone, regarded as half a person without his Dom--and Tony thinks that life is normal.
Or at least, he thinks its normal until he meets Bucky and Steve.
Bucky has more freedom than Tony could imagine and Steve's only wish as Dom is to take care of his subs, and when both Dom and Switch fall for Tony, they make it clear there's room for him in their arms and hearts.
First they have to deal with Ty though, first they have to free Tony from an abusive Dom and a history of hurt, and that won't be easy.
But Tony's ready. He's ready to move on and reclaim his life, ready to take a chance on something new, and ready to learn the beauty of willing submission with Bucky, and the truth of a caring Dom in Steve.
Unbeknownst to the general public, the serum gave Steve tentacles. Bucky never blinks an eye at them, but it does become an issue when they get together with Tony. Steve can't parse what the hell Tony is thinking when he stares at his tentacles, but it can't be anything good.
Steve is very wrong about that.
Or: Steve and Bucky have never heard of tentacle porn and Tony despairs at their ignorance.