From what I remember it was a pretty long fic. It was part of a series. I had read the first part and bookmarked the rest (then my computer died).
At the end of the first part, Tony sees Steve and Bucky kissing and leaves. I think it was a goodbye sorry of kiss. When the avengers realise Tony had gone, I think Natasha says something to Steve about how Tony's been waiting for Steve to dump her and when she saw them kissing her worst fears came true.
I'm sure it was it was a girl!Tony fic but I could be wrong. The series may have become Steve/Tony/Bucky as it went on.
I'm not sure if I've imagined this or if I'm combining a couple of different fics but I've been looking for a while with no luck.
I'm pretty confident that I read it on ao3 but I've searched through the girl!Tony tags as well as the Steve/Tony/Bucky tags with no luck.
Can anyone put me out of my misery?