It appears that there is more than one choices for x_t to 3dm conversion, as several 3D formats use 3dm extension for their file types. Anyways, this looks like a 3D format conversion and as such you can do it with some dedicated 3D/CAD conversion tools like PolyTrans or similar programs.
Open X_T file Open 3DM fileAn online CAD conversion service
A 3D CAD translation software