this has been asked many many times, and the technical answer is that 3d flying is flying beyond the stall.
in most 3d manoeuvres the wing has little or no forward speed and the control surfaces rely on propwash to remain effective.
3d manoeuvres include- the waterfall, prophang(hover), parachute/elevator, torque roll, flat spin, pinwheel, blender, harrier and the list goes on, and on, and on!

what makes a 3d plane?
in my opinion, reasonably low wing loading combined with a good 3d airfoil. lots of power (at least 1.5:1 power to weight ratio).
large control surfaces with a lot of throw, and most important- a good pilot