We have found one software suitable in our database for this conversion.
It looks like some dedicated CAD / 3D modeling converters such as CrossManager, support both 3DM (3D NURBS CAD) and CGR (CATIA MultiCAX) files and can convert one to another. However, we couldn't really verify the reliability of this information.
Updated: August 9, 2016
A dedicated CAD converter with a large file format support that allows users to convert (translate) CAD designs or drawings between different formats. CrossManager is a commercial product, you will need to purchase a license. It is available in English, French, German, Italian, Czech, Chinese, Russian and Korean language.