A 3DM file is a 3D graphics file format used by Rhinoceros, a popular 3D modeling software. To open a 3DM file, you can use Rhinoceros itself if you have the software installed. If you don't have Rhinoceros, there are several other options available for viewing 3DM files, such as the Rhino 3D software, AutoCAD, and 3D-Tool. Additionally, you can use online 3DM file viewers like A360 Viewer or GrabCAD. These tools allow you to open and view 3DM files without the need to install any specialized software. With these options, you can easily open and view 3DM files regardless of the software available to you.