Dungeon Siege III isn't anything like either of the other two games in the series -- which makes sense since Dungeon Siege II wasn't anything like Dungeon Siege, either. The main difference here is that Dungeon Siege III was clearly designed with consoles in mind, to the point where it was barely playable using a keyboard and mouse when it first came out. Fortunately, Obsidian quickly released a patch to make the controls at least passable on the PC.
In the single player game, you control one character, and you also get a companion (one of the other three characters, once you've met them during the campaign) who is under computer control. For multiplayer, you can choose between an offline two-player mode (where the second player takes over for your companion) and an online four-player mode.