A .3dm file is a 3-D image file associated with the Rhinoceros program. A .dwg file is a technical drawing file associated with the AutoCAD computer-aided drafting program. A .3dm file can be converted to .dwg format using the appropriate application.
Download and install Sycode 3DM Import for AutoCAD (free trial, see Resources section). This AutoCAD plug-in adds a new menu to the AutoCAD program, allowing easy import of .3dm files. The plug-in is compatible with AutoCAD versions through 2011.
Launch AutoCAD, and open the .dwg file you wish to import the .3dm file to (or open a blank .dwg file).
Select "Import" from the "3DMImport" menu, then browse to the .3dm file you wish to import, and click "OK."