Are you looking for a specific kind of relationship?
PurpleCatGirl writes pretty good stories, primarily
this one, though... well look through the genres and tags and decide for yourself whether that's something you're interested in.
Also
We're Not So Different, You And I is an amazing story with a poly relationship. Same CWs apply.
If you're looking for something more male-oriented, then you'll have to turn to someone else cause I've never read those.
Edit: Actually, I don't have experience writing poly relationships, but I think what you'll want is not to focus too much on it. That may sound weird, but coming from the idea that three times the interactions (for just three people, and it's gonna grow fast for more) means three times the opportunities for drama you don't need.
I think, rather than trying to detail their relationships and feelings, you should focus much more on the surface and on the daily interactions. There will always be some drama behind the scenes, but jealousy or possessiveness realistically won't lead to any serious conflict in a poly relationship - or it will no longer be a poly relationship. Anyone entering a poly relationship wants to be there, so it's a given they won't flip out because of envy or something. If they don't want to be in such a relationship, or if they don't know it's one, it's an immediate red flag.
totally not me finishing this and realizing the same things apply to almost any other relationship