Opening a 3DM file is essential for working with 3D modeling and CAD projects. Here are the steps to open a 3DM file in popular CAD software:

1. Rhino: If you have Rhino software installed, simply double-click the 3DM file to open it in the program. You can also open Rhino and use the 'File' > 'Open' option to locate and open the 3DM file.

2. AutoCAD: To open a 3DM file in AutoCAD, go to the 'File' menu and select 'Open'. Navigate to the location of the 3DM file, select it, and click 'Open' to load it into the program.

3. SolidWorks: In SolidWorks, go to the 'File' menu and choose 'Open'. Browse for the 3DM file, select it, and click 'Open' to import it into the software.

4. Fusion 360: To open a 3DM file in Fusion 360, go to the 'File' menu and select 'Open'. Locate the 3DM file on your computer, select it, and click 'Open' to bring it into Fusion 360.

Once you've opened the 3DM file in your preferred CAD software, you can start working on your 3D modeling project. Keep in mind that some CAD software may require plugins or additional steps to properly import and work with 3DM files.

It's important to note that not all CAD software supports 3DM files natively. If you encounter issues opening a 3DM file in your CAD program, consider converting it to a more widely supported format like STEP or IGES using dedicated file conversion tools.

By following these steps, you'll be able to open and work with 3DM files in popular CAD software, allowing you to create, edit, and manipulate 3D models for various projects and industries.