My experience is that there are a lot of people who tell you "how it's done" and they get very weird about the rules. I understand that comes from their experiences and the general aversion of wanting to not be associated with "poly couple hookup types" you see on Tinder or wherever.

I am poly, but typically just date solo. I've joined poly couples as a third, but it's never really felt right for me; I don't like hierarchical dating.

My dream relationship is a triad, but my god is that hard to find. I just want my little throuple commune in the woods. :3