ResidualVM Forum said:.3DO FILES (3D Objects)
These files are in an ancient 3D format that is difficult to open in modern 3D modelling programs. Thankfully we have command-line tools written by cplhenshaw that convert .3do files to the commonly used .obj format and back again.
It can be downloaded here: http://www.mediafire.com/download.php?g589e8eehnp1bai