All U RC Pilots and not one good simple answer..............

Flying on the Prop rather then Flying from the Wing is a simple definition of 3D.

All Airplanes fly because of the low Pressure on the upper wings surface. In 3 D the Wing is no longer the primary source of lift in all maneuvers the Engine Thrust is.

The reason Full Scale Aircraft don't do 3D is lack of Power. Our models typically have 2 times or greater to Fly 3D.

I can go on and but I think U will get the idea.

Ian