I honestly don't think that I have technically compromised on anything in the 3 years I've been in poly relationships. But that's not because I'm super assertive-- the opposite. I'm very easy going on things that don't matter to me, and I try to be thoughtful about my partners' feelings on things that do matter. When there's something that really matters, and it matters to the point that I want to go against the preferences of a partner, they've always sort of sat up and listened when I state my case, and let me have what I need.
I can see some areas in the future where compromise is needed (if I move in with two of them, there are a lot of opportunities for our preferences/wants to conflict), but I think it really comes down to telling the difference between wants and needs, and working cooperatively to make sure everyone's needs are met, and that their wants are met as much as possible.
Feelings (acknowledging them, expressing them) tend to go in the needs category for me.