Having tried all three, its a stark difference in how much more social Lemmy is comparatively. Its not even close. Almost all posts I've encountered on lemmy have interaction; whereas, more often than not, posts on the other two platforms have no interaction. Wonder what the driving factor is behind this difference?
I've never heard of Nostr but Mastodon is a twitter clone and I don't find that style of website suits discussion well since you subscribe to accounts rather than communities.
It's an interesting dynamic!
I find myself talking more on lemmy as others say because it's easier/made for talking about topics. Mastodon and other fedi services center around following the account that made a thing rather than the thing(s) themselves. And that's fine, both have their place.
I think the other aspect is the easy to follow discussion threads. IMO it's the cleanest way to show and follow branching discussions.
I do like how it "looks" the most on topics. I wish mastodon had something similar revolving around their posts/hashtags.