A .7z file is ; it is a compressed archive (like a .zip file, but better). A .3ds file is the legacy 3D Studio DOS mesh format.
Crucial Note: If the archive only contains a .max or .blend file, you cannot save it as .3ds without first opening it in its native software. You need a decompression tool. Windows cannot open .7z files natively. convert 7z to 3ds
If you’ve just downloaded a 3D model pack and found a mysterious .7z file instead of a .3ds file, don’t worry—you haven’t downloaded the wrong thing. A .7z file is