3dm is not a file of 3ds max, but a format of Rhinoceros software. If you want to open it in Rhino, if you want to use it in max or c4d, you can convert it into formats such as obj or fbx in Rhino.
You can open it directly with WordPad to view, edit and modify it. In addition, there is a binary file format (*. MOD) is related to this, and it is not published as a patent, so it will not be discussed here, but maya can also compile obj files.
Basic structure:
OBJ files don't need any kind of file header, although they usually start with a few lines of file information comments. An OBJ file consists of text lines, and comment lines begin with the symbol "#". You can add spaces and blank lines to the file at will to improve the readability of the file.
A line with words begins with one or two marker letters, that is, keywords, which can explain what kind of data this line is. By adding a connector (\) at the end of each line, multiple lines can be logically connected to represent one line. Note that spaces or tabs cannot appear after the connector (\), otherwise it will cause file errors.