There is only one file type that uses the .U3D extension:
Universal 3D File
3D CAD file saved in the Universal 3D File Format; standardized by Ecma International as the ECMA-363 standard; contains 3D model information, including shapes, textures, lighting, shading, and motion data; supported by many CAD and 3D graphics programs.
The first edition of the Universal 3D File Format was standardized in December, 2004. The second version, August, 2005. The third edition, June, 2006. And the fourth edition, June 2007.
Updated: September 10, 2018