Opening a 3DM file can be done using various software programs, the most popular being Rhino, a 3D modeling software that supports the 3DM format. Here are the steps to open a 3DM file using Rhino:

1. Launch Rhino on your computer.

2. Click on the 'File' menu and select 'Open' or use the shortcut 'Ctrl + O'.

3. Navigate to the location of your 3DM file and select it.

4. Click 'Open' to import the 3DM file into Rhino.

Aside from Rhino, other CAD software such as AutoCAD, Fusion 360, and SolidWorks also support the 3DM file format. You can follow similar steps to open a 3DM file in these software programs.

Additionally, there are online 3DM file viewers available that allow you to upload and view 3DM files without the need for specific software. Simply search for 'online 3DM file viewer' in your preferred search engine and choose a reputable option.

In conclusion, opening a 3DM file can be achieved using dedicated 3D modeling software like Rhino, as well as other CAD programs and online viewers. Knowing how to open 3DM files gives you access to view, edit, and work with 3D design files created in Rhino and other compatible software.