http://www.netfabb.com/stl_repair_fixing.php
With 3D Studio Max 7 or above, it's very simple to convert the .3ds format files to .stl format.
- click File button on the menu bar, then click export
- In the export format menu choose STL format
- click export button to finish
Note: You must choose the object you wish to print in the main window, otherwise you may get a warning message indicates no object selected to export.