rod03801: Instead of it being negative, as in moving backwards to normal Backgammon flow, why not have the third die as one that the player can use in either direction at his option?
As for rolling triplets, I'd just count it as double instead of triple, so it'd give you six of the number rolled. That'd still be a good roll. I'd also not count doubles using the third die, just count them when the doubles use only the original two dice or there'll be way too many doubles in the game.