An A-Frame is when three people (usually two guys and a girl) engage in a sexual sort of threesome. Usually the two guys stand facing each other with the girl between them. With the girl still in the middle, she is giving one of the guys head and the other has himself inside of her. They do this at the same time, switching terms and positions. Sometimes the two guys place their hands over there head towards each other (like in an "A" shaped way).
Since I can't really show you a picture of an A-Frame, the definition pretty much sums it all up.