I was thinking that the 3rd subclass slot was for when you locked in one of the original ones. I was thinking you could only lock in one of you subclasses (and you get another level one subclass of that type) so you don't have 30 or so builds that could accommodate every situation you encounter. However I must admit that it does look like an arc ability in the trailer at 5:24. Does anybody know if in the beta if you had 10 slots in your inventory for subclasses just like weapons and gear? That could help disprove me theory. I hope that there is a 3rd subclass but I don't want it to be a congrats you reached level 30 now you have the by far best abilities in the game. It would be neat if they were more cooperative minded to help in raids and other end game activities.