The most obvious choice for any skp conversion would be to use SketchUp, because it is the program that natively uses .skp files. But export to 3ds format appears to work only in the commercial and not the free version of the program.
Alternatively, 3ds Max should support SketchUp graphics by default, so this presents also a choice for skp to 3ds conversion.
In case you do not have access to SketchUp or 3ds Max, you will have to rely on some 3rd party alternatives that offer support for SketchUp formats.