I am a little confused on this third class as well. I am passed the point of getting the third class, have completed the quest, and even spoke to the druid master to select the third class...
What I don't understand is that when I go to the house maid I can see all three class s/r/d, but when I play I only get skills from two of the classes, not the third... Am I missing something?
Is the third class really just an option to swap in for a second class? what am I missing?
Nope, having a third class just means you have another option to pick from. In my case, my 3rd class is warrior. So I can only be d/s, d/w, s/d, w/d, s/w or w/s. You don't get any extra abilities from the class you don't use in your combo. You can't have all 3 abilities from all of your classes at once.