You can use the FBX intermediate format to convert AutoCAD or Inventor 3D models to .DAE files or .DAE files to AutoCAD DWG or Inventor models. Alternatively, the applications Autodesk 3ds Max and Maya support direct export to the Autodesk Collada format.
For FBX, use the current version of the free application "Autodesk FBX Converter" (see www.cadstudio.cz/download or www.autodesk.com/fbx). With the FBX Converter you can convert your 3DS or DXF file (both can be exported from AutoCAD - see 3DSOUT or DXFOUT) to a .DAE (Collada) file -- first convert to FBX then from FBX to DAE.
The opposite way is to convert DAE to FBX and then FBX to 3DS or DXF (both can be imported to AutoCAD - see 3DSIN and DXFIN).
DAE files can be used in multiple software tools for Playstation, nVidia, Unreal, Google Earth, Google SketchUp and other applications.