how do I convert 3DS to DWG, DGN or similar files?

try to open it up in 3d viz or max and export as dwg ( not sure of this)
otherwise use OLE or import in autocad itself ( it is likely to work)
BUT THE BEST TOOL YOU CAN USE IS OKINO POLY TRANS SOFTWARE. Google it to know more of what it does.Im in the office now , when I get home I will try to work out a direct solution to your question , but no doubt Okino will do it.