How to convert 3ds to step?

I use a Simulator Soft to create PCBs. This soft can create a 3D view and can save it as a .3ds file. I use Rhino to modify the created .3ds but Rhino is unable to export .step.
I need to convert the .3ds to .step but all converters I have tried are absolutelly usefull.
Any suggestion?.
Thanks for your time and attention.