Opening 3DM files requires specialized CAD software such as Rhino, AutoCAD, or other 3D modeling tools. Here are the steps to open a 3DM file using Rhino software:

1. Install Rhino: If you don't already have Rhino software installed on your computer, download and install it from the official website.

2. Launch Rhino: Once the software is installed, open the Rhino application on your computer.

3. Open 3DM file: In Rhino, go to File > Open or use the keyboard shortcut Ctrl + O to open the file explorer. Navigate to the location where the 3DM file is saved and select it to open.

4. View and edit: Once the 3DM file is opened, you can view and edit the 3D model using Rhino's comprehensive set of tools and features.

If you don't have access to Rhino, you can use other CAD software such as AutoCAD, SolidWorks, or other 3D modeling tools to open 3DM files. Each software may have slightly different steps for opening 3DM files, so refer to the specific software's documentation for guidance.

Additionally, you can use online 3DM file viewers or converter tools to open and view 3DM files without needing to install specialized CAD software. These online tools may provide limited functionalities compared to dedicated CAD software, but they can be useful for quick viewing of 3DM files.

In conclusion, opening 3DM files requires specialized CAD software such as Rhino, AutoCAD, or other 3D modeling tools. By following the steps outlined above, you can easily open and view 3DM files for your design and modeling projects. If you encounter any issues opening 3DM files, refer to the software's documentation or seek assistance from the software's support team.