file extension 3DM

Help how to open 3DM files:

To open or to view 3D graphics or CAD-CAM-CAE file types,  you can use one of the suggested application listed below the extension description. If the application is no longer available, try to use a very popular software suite from Autodesk, such as AutoCAD for CAD-CAM files, or 3D Studio MAX for 3D graphics files. AutoCAD and 3D Studio MAX support plugins (to provide new functions and features), which can be used to add import/export support for other 3D file formats in AutoCAD or 3D Studio MAX.
Other very popular application is Blender. The Blender is free open source 3D content creation suite, available for all major operating systems (Microsoft Windows, Linux, MAC OS X, Solaris, Irix), which also provides basic support for many 3D formats and plugins.
 
This is a general information about how to open the most common 3D file types, but there are some standards or 3D file formats, that have to be used  with specific applications only. Many of the 3D formats were discontinued years ago and it's very hard to find any application, that can open or convert these old formats.

How to convert file with extension 3DM:

The general information about how to convert 3D files (if there is no suggested converter application) is pretty simple, because most of 3D applications can also import, export, open or save files in other formats too. When the file is open, you can simply use the SAVE AS../EXPORT function in the FILE menu, and select the 3D file format you desire and just wait for the application to finish the saving/exporting. There is plethora of online converters available on internet for the most well-known 3D formats (AutoCAD, 3D Studio MAX, Blender, Alphacam, Pro/ENGINEER, Pov-Ray etc.).