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.