Opening a 3DM file can be done using CAD software such as Rhinoceros 3D, SolidWorks, and AutoCAD. These programs are compatible with 3DM files and allow you to view and edit the content. If you don't have access to these software, you can also convert the 3DM file to a more common format such as STL or STEP using online converters or specialized software. Make sure to choose a reliable converter to ensure the integrity of the file. With these options, you can easily open and work with 3DM files for your design and manufacturing needs.