I'd try to join place that has lots of people, ignoring all the ideological fights or other kinds of backstage dramas and focus on football, people who like it.

I second the other comment just made in reply to yours. I also want to add that this would kind of happen on Reddit as well where occasionally there’ll be more than one community for the same topic. Sometimes it’s different themes (there’s standard mature f1 and there’s the f1 that just wants to shitpost and meme). Sometimes one community exists but hasn’t gotten enough attention or growth moderation so people join a more well managed one. Sometimes one community exists for general purpose discussion and one for more specific stuff (like cycling vs randonneuring)

I feel like similar things will happen in the forum fediverse (it’s what I call the fediverse alternatives to Reddit). There’s going to be a period of thrashing and churn. And then it’ll settle down with each community having its own vibe and we members get to try it out and settle where we want.

> I want a community for my football club, but there are already 3 across multiple instances - which one do I join?

I would use the community browser at https://browse.feddit.de/ to find the most populated and active community, and stick with that one. If the other 2 aren't very active, maybe make a post in those communities letting them know there's a more active community at X.