Here's one solution.
Blender (http://www.blender.org/) can open .3DS and .OBJ formats.
File Formats
Blender comes packed with import/export support for many different programs.
3D
3D Studio (3DS), COLLADA (DAE), Filmbox (FBX), Autodesk (DXF), Wavefront (OBJ), DirectX (x), Lightwave (LWO), Motion Capture (BVH), SVG, Stanford PLY, STL, VRML, VRML97, X3D.
And this AC3D Exporter plugin for Blender will allow you to export to .AC.
Export selected meshes to AC3D's .ac file format.