2 of those accounts watch 1000+ people. I generally suspect bots or people looking for Thank You shouts.
One looks like a new account, probably just using keyword searches to find things they like.

I don't know... I've been followed by serial adders before (whom I generally don't acknowledge) and for a couple of reasons these don't quite fit the pattern, particularly in that I'm still among the most recently added accounts under "Is watching" on both of the two that watch over a thousand. None of the three of them watch each other. The closest thing to a pattern I've seen between them is that all three seem to favor gay male porn, which is pretty common.