I am sure your going to get some varied answers on this one. Here's mine. Quique(IMHO) invented 3D. He called it flying beyond the stall. This is a good description. The first time I saw Sean McMurtry do a 3D flight I was awestruck. I can remember my group of flying buddies standing there wide eyed. The plane seamed to be defying the laws of physics and gravity.
So I would say 3D is controlled flight with the airplane stalled.

david