I recommend Ultimate Unwrap from file format conversions. That only costs $35 from www.unwrap3d.com
I haven't really tested this so I can't vouch for it, but a free converter I know of is Crossroads3D. Or use Wings3D; it can import obj and export 3ds.
I haven't really tested this so I can't vouch for it, but a free converter I know of is Crossroads3D. Or use Wings3D; it can import obj and export 3ds.