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?
Well Mastadon is good for screaming into the void and hope someone shouts back. Lemmy is kind of like a forum type community where you already know someone is going to like your topic if it's in the right sub.
It's good for small hobbyist communities that get built up from IRL spaces or broader online collaborations. If I've got a school group or hobbyist club and I want a bespoke "members only" social media space, Mastadon works great. Like Discord without all the obnoxious pop-in "Would you like to give us $40/mo for glittery icons?!" nitro ads.
Reddit-brain is all over Lemmy. This is a far cry from the technical focused communities you'll find on Github or StackExchange.
Have you used StackExchange? It's very much "call you an idiot for doing things in a different way."