OpenNURBS (Non-Uniform Rational B-Splines) is an open source toolkit for reading and writing models in the 3DM format. It allows other applications to exchange 3D geometry data with Rhino 3D software. You can do the following with 3DM files:

• Attach them as references
• Open them in read-only mode
• Open and save them as DGN files

There are several options available when you open or import a 3DM file.