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 think the answer is fairly clear. Lemmy's topics & votes system funnels condenses the user-base to focus on particular things at particular times. The total number of users may be smaller than Mastodon, but basically everyone on lemmy is looking at the top posts on the front page first, and then exploring to other stuff later; whereas on Mastodon everyone is just doing their own thing.
Focusing people on one topic means that there will be discussion at that topic at that time; and discussion leads to people checking back to read and reply to responses...
I routinely use both Mastodon and Lemmy. I see a lot more varied content on Mastodon, but it is more fleeting. i.e. very little discussion, and fairly short window of interaction with posts. Lemmy has a lot less 'stuff', but a lot more conversation.
I think the difference is interesting, but it definitely isn't something we should use to say which platform is doing better or anything like that.