3D
.3D is a model format developed by Epic Games for the first version of the 
Unreal Engine.
Models are divided into two separate files: the data file containing surface data (filename_d.3d) and the aniv file containing vertex positions (filename_a.3d).
Most features of the format (e.g.: per-poly render styles) are currently unsupported due to technical limitations. Both standard Unreal models as well as higher-precision models from Deus Ex can be loaded.
The format has certain limitations that would make it impractical for original asset production, such as UV coordinates being a single byte each, or the fact that normals are not stored and must be calculated at runtime by the engine, potentially causing stuttering or longer load times. Its use is therefore only recommended for already existing models from Unreal Engine 1 games.