Most of the last few girlfriends like it with thumb on the clit, two fingers on the G-spot and the pinky in the butt. The trick is to get all of them to move in the right way. Takes a little practice.
You don't need to do this long before you switch to tongue on the clit and two fingers on the G-spot and thumb of the other hand on the butt. Then your face gets a bath!
My current girlfriend isn't crazy about the finger in the butt unless she's very horny, like after a couple of joints and a vodka. She's got some bad memories of a previous BF who used the butt a little too much.