You can easily import old 3ds files to SketchUp using the default File ► Import menu option, which should allow you to convert .3ds graphics to .skp files. More about 3ds to skp conversion (import/export of 3ds) can be found on SketchUp help site.
You will however need the professional version of SketchUp if you want to save the file back as .3ds files.
Updated: September 7, 2022