3DM is a format for 3-dimensional data described by so-called NURBS. These Non-Uniform Rational B-Splines let you create all kinds and shapes of 3-dimensional objects in a 3D editor. But 3DM files can include Surfaces, Meshes and Solids.
The 3DM format was created by the openNURBS Initiative, founded by Robert McNeel & Associates. McNeel is the company behind Rhino 3D, a NURBS Modeller program used in many different design areas. The 3DM format is therefore often referred to as Rhino 3D format.
The openNurbs Initiative offers free programming libraries so that any 3D software manufacturer can implement the 3DM format. The 3DM format should be a universal data exchange format for 3D data. So far, however, most of the 3D applications can only be equipped with a paid (Sycode 3dm, Rhino.Io) plugin to read and write 3DM files.
Alternatively, you can also use a converter like Okino PolyTrans. Since there are now 4 versions of 3DM (through the advancement), the success of an import by a plugin is never 100% guaranteed.
Most often resolving problems with opening 3DM files is very simple. Just install an appropriate program that supports such files. All of the listed programs support 3DM files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of 3DM files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate 3DM files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Rhino OSX applications is not on the list, select "Browse" option in order to manually locate the directory where Rhino OSX has been installed.
Sometimes problems with opening 3DM files may not lie with the application itself, but can arise due to other causes, such as: