Opening a 3DM file can be done using various CAD software programs, with Rhino being one of the most popular choices. To open a 3DM file in Rhino, simply launch the software and then use the 'File' menu to select 'Open' and browse for the 3DM file you want to open. Another way to open a 3DM file is to use another CAD software that supports this file format, such as AutoCAD. You can also use online file viewers and converters to open and view 3DM files without needing to install any software. Some of these online tools allow you to upload the 3DM file and then view or convert it to another format for easier access. Overall, opening a 3DM file is quite simple and can be done using various software and online tools.