.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.