There is only one file type that uses the .MD3 extension:
Quake 3 3D Model
Three-dimensional model format used by Quake III Arena, a first-person shooter game; used primarily for player and character models; includes frame, vertex, surface, and shading data; also contains animation data that defines the possible movements of the model.
MD3 models are also supported by Sauerbraten, which uses the Cube game engine.
Updated: August 27, 2010