A "7z to 3ds converter" isn't quite what it sounds like at first. You're not magically transforming a compressed archive into a playable Nintendo 3DS game or a 3D model all at once. That's a common misunderstanding. A 7z file is an archive—a single, compressed container that holds one or more files—while a 3ds file is a completely different beast: a 3D model format used in Autodesk 3ds Max. A true "converter" between these two doesn't exist, because you're not converting one file type into another; you're extracting content from one container to prepare it for use in specialized 3D software.
Keka charges a small fee on the Mac App Store (though it is free on their official website). 4. ZArchiver (Android) 7z to 3ds converter