If you want to import a 3DS-file into AutoCAD then let us know what version you have, imho was AutoCAD 2008 the last version having the 3DS-import command built in. Later on there was an ARX available but not for the current versions of AutoCAD.
If you have 3DS Max (or 3DS Max Design) you can import 3DS-files directly, no need to develop anything.
As external option: there is no ready to use viewer from Autodesk that can show 3DS-files and be customized with dotNET (AFAIK). So I would recommend to look to something like >>>irrlicht 3d-engine<<< or >>>ogre<<< or any other viewer that is able to load/show 3DS-files and has an .NET-based API or source-code modifyable.
Good luck, - alfred -