Personally I use blender. It can import alot of file formats and can export directly to .mdl with ventilators python script... However Im not sure if they ever fixed the .3ds import as of yet. Another option might be wings3d. Im sure theres a ton of other ones available too.

You can get blender from blender3d.org

http://www.freewebs.com/otama_syndicate/index.htm - Each master to his own technique.

- Not me said the bee, Nor I said the fly.