You can multiclass almost as much as you want. On each level gain, you can choose to increment the current class, or add a new one. Just read up on the possible penalties if the classes get too far apart.
Keep in mind, though, that a triple class character won't advance very much. Average finishing level for normal mode is 16. With even advancement, that's only 5/5/6 for a triple.
Nightowl2
|