Flying 3D is flying an RC aircraft to its extreme limits. It is basically keeping a plane airborne by the thrust of its prop, not the lift from it’s wing. A 3D flyer keeps his aircraft at a near stall or “High Alpha” condition. To do this requires a plane with a very high thrust to weight ratio. A classic example is a “Hover”. This is basically hanging an aircraft from it’s nose and flying vertically, with no sideways, or lateral movement (like a helicopter).
In earlier days this type flying wasn’t called 3d flying, it was commonly called “Hot Dogging” or “Free Style” flying.
3D flying takes a lot of practice and is generally considered “advanced” flying. It can be very unstable. It takes a mastery of all flight regimes and a keen ability of flight control skills. Getting an airplane to hover is a lot like trying to balance a pencil on the end of your finger. It takes constant power, rudder, elevator, and aileron corrections to keep it steady.
There are variety of 3D type aircraft on the market. Indoor/outdoor flyers, foamies, profiles, balsas etc. 3D can be done at the field, a local park, or even in a gymnasium. The internet is a major source of information.