This depends a little bit on each game:

Trine 1, 2 and 3 - online multiplayer works with 2 or 3 players (no 4 player support)
Trine 4 and 5 - online multiplayer works with 2 or 3 or 4 players (in Unlimited Mode where every player can be any character they want)

In all of the games you can mix and match the players as you wish (hot join), so you can start playing alone, then start an online game and have a friend join, and you can later have a third friend join (and leave, too).

Hope this still helps despite the delay in posting!


Best,
Joel
Frozenbyte team, developers of the Trine series