ChimeraX does not export .3mf format and we do not plan to support it at this time.  There are dozens of 3D scene formats and we don't have the resources to support lots of them.  We have looked at .3mf in the past but most of our scene export effort has gone into GLTF.  The STL format is in a totally separate category since it has no support for colors and is simply a set of triangles.  That is suitable for single color 3D printing.  For a list of the scene formats ChimeraX exports see the save command docs section under 3D objects (currently gltf, stl, obj, x3d, vrml, vtk).