What is a 3ds file ? For what I know at this time, it's a 3D file. It can be opened by any program managing this kind of file, like Anim8or (free software), to see to what the objects looks like.

I don't have any idea of how to make them, I still miss the needed knowledge. :-(.

It is meant to be putted in "Models" directory, and the command you have to write into ssc files is placed under "Mesh" section.

Mesh "asteroid.cms"

Another kind of 3D file, named cms, is only used by Celestia.