In the fanfiction, it could be that there was some sort of mission or adventure that brought the three together. During that time, the relationship developed and the girl got pregnant. Steve and Bucky might then use their skills and resources to protect the girl and prepare for the baby. They could have to deal with enemies who might want to harm the girl because of the baby or their relationship.
It could also be strained. There might be some jealousy or differences in opinion on how to handle the situation. For example, one of them might want to be more involved with the girl during the pregnancy while the other feels left out, which could cause some tension between them.
In this fanfiction, perhaps Steve and Bucky come across Tony when he's being bullied in school for being a genius. They see his potential but also his vulnerability. So, they adopt him. As time passes, Tony starts to show an interest in Steve's shield and Bucky's combat skills. They encourage his inquisitive nature. Steve cooks for them all, creating a homey atmosphere. Bucky teaches Tony how to deal with nightmares as he has his own experiences. And Tony, in turn, uses his intelligence to improve their living situation and help them in their own missions in some way.
The plot could start with Steve realizing that his bond with Bucky is stronger than his relationship with Tony. Maybe there have been some misunderstandings or conflicts between Steve and Tony that make Steve reevaluate his feelings. Then, Steve decides to break things off with Tony in a rather dramatic way. After that, it could explore how Tony reacts to this rejection, and how Bucky fits into the new dynamic between Steve and the rest of the Avengers.
At first, it might be a bit rocky. Tony, being used to his own space, might feel smothered by Steve and Bucky's over - protectiveness. But as time goes on, he'll start to see their good intentions. Steve's moral compass will inspire Tony to be a better person, and Bucky's loyalty will make Tony feel secure. Eventually, they'll become like a real family, with Tony looking up to Steve as a father figure and seeing Bucky as a cool older brother.
There's also 'Forging New Bonds'. This fanfic focuses on the post - conflict relationship between the three. It explores how they start to understand and accept each other better. The writing style is quite vivid, making it easy to picture the scenes in your head.
One common theme is loyalty. Steve and Bucky have a deep loyalty to each other that is often explored. For example, in many fics, Bucky will go to great lengths to protect Steve and vice versa.
I really like 'Bucky's Redemption'. In this fanfic, Bucky is on a journey to fully come to terms with his past actions while Steve is there to support him every step of the way. It shows their loyalty to each other in a very touching way. There are some intense emotional moments as Bucky struggles with his guilt and Steve tries to pull him out of the darkness.
Typically, Steve and Bucky lemon fanfiction often emphasizes their deep emotional connection. It might show their loyalty to each other in different situations. For example, in some fics, they support each other through post - war traumas.
Maybe Bucky kidnaps Steve because he believes it's the only way to protect him. There could be a new and powerful threat that Bucky has discovered, and he thinks that by taking Steve away from the public eye and the Avengers, he can keep Steve safe. But Steve doesn't understand Bucky's motives at first and there's a lot of tension between them until Bucky can explain.
One idea could be a time - travel plot where Darcy accidentally gets sent back in time and meets a young Steve and Bucky. They end up on an adventure together, with Darcy's modern knowledge helping them in unexpected ways.