If you work with design or modeling software, you may encounter 3DM files frequently. These files are commonly associated with CAD (computer-aided design) programs and are used to store 3D model data. Opening 3DM files requires specific software that can read and edit this file format. Here's how you can open 3DM files:

1. Rhino 3D: Rhino is a popular CAD software that can open and edit 3DM files. If you have Rhino installed on your computer, simply double-click the 3DM file, and it should open in the Rhino interface. You can then work with the 3D model as needed.

2. AutoCAD: AutoCAD is another CAD program that supports 3DM files. To open a 3DM file in AutoCAD, go to the 'Open' dialog, navigate to the location of the 3DM file, and select it. AutoCAD will then display the 3D model, allowing you to make changes or annotations.

3. SketchUp: SketchUp is a 3D modeling software that can also work with 3DM files. To open a 3DM file in SketchUp, go to 'File' > 'Import', and select the 3DM file from your computer. The 3D model will be imported into SketchUp, and you can start working with it right away.

4. Autodesk Fusion 360: Fusion 360 is a cloud-based 3D CAD/CAM tool that supports 3DM files. To open a 3DM file in Fusion 360, go to the 'File' menu, select 'Open', and choose the 3DM file from your computer. Fusion 360 will then load the 3D model, allowing you to view and edit it in the software.

5. FreeCAD: FreeCAD is an open-source parametric 3D CAD modeler that can open 3DM files. To open a 3DM file in FreeCAD, go to the 'File' menu, select 'Open', and browse to the location of the 3DM file on your computer. FreeCAD will then load the 3D model, and you can start working with it.

In addition to these software programs, there are also online 3DM file viewers and converters available that can help you open and work with 3DM files without installing any software. By using these tools, you can view, edit, and convert 3DM files as needed for your projects.