A 3DO (or "3D Object") file contains the geometry data of 3d models that are rendered in NR2003 (not including track geometry).  These files are are scene graphs representing a single object to be rendered.  When referenced by a PTF file, these objects will be rendered at the location specified by the PTF file, but without any collision detection.  3DOs are also used to represent a car's visual geometry, which may or may not match the physical geometry hardcoded into NR2003.  3DO files may be as simple as a single, static mesh that is always visible from any distance to very complex systems that are only visible under very specific conditions or swaps-out different versions based on very specific sets of conditions.  3DO files are structured as a node tree (usually referred to as a scene graph) with a single root node.