Looking to grab the trilogy, not just for Multiplayer though but for single player as well, is single player good?
You will find a game easily (Europe/North America region) all the time , even low ranks only games , except at 8:00-11:00 AM GMT.
KZ2 Has around 210 players online , there is no counter in KZ3 though.
Single player? Multiplayer is where it's at.
KZ1's single player's good, I don't even remember KZ2's other than the bosses. KZ3's not bad, but not that great. Just keep your expectations on the multiplayer, unless you care about getting all the trophies.