As far as i know there is no "export to .3ds" option in Inventor.
But i did find a working plugin for Inventor 6 and 3dsmax5 (and 3dsmax3 and 4 as well) - you can download it here:
http://adeskftp.autodesk.com/prodsup...Max_Plugin.zip (and the installation instructions are found here:
http://www.mekvizion.com/Support/Max...gin_Readme.txt )
Hope this helps