Here is the process for converting from .skp to .3ds:
1) Download or create a .skp model in Sketchup
2) In Sketchup, export your model to “Google Earth 4 (*.kmz)
(note: be sure to choose version 4, because the other one is different)
3) Change the extention of the kmz file to .zip
4) Pull out the .dae file
5) In FBX Converter, convert the .dae file to .fbx
6) In FBX Converter, convert the .fbx file to .3ds
http://forums.devshed.com/game-development-141/converting-sketchup-models-to-3dst-519710.html