I'm going out with this girl but...?

Easy, just find a time when its just you and (one of them) and talk to them and tell them what you just told us. Start by asking if he ever liked her and if he did then tell him that although you feel guilty, she is your girlfriend. And say that you still want to be friends with him. Then repeat with the other friend. Do not talk to both of them at the same time; thats a recipe for trouble. And if you are talking to one of them and they say that they never liked her, then just say that you are her boyfriend now and make it no big deal. Good luck (: