If you want, I can provide a ready-to-run Blender Python script for batch converting extracted model files to .3ds or a concrete command-line sequence for your OS—specify which OS and which exact conversion target (Autodesk .3ds model or Nintendo ROM).
If you are trying to convert a game file from .7z to .cia for installation on a Nintendo 3DS: Extract the file from the .7z archive first.
There is no software that converts .7z to .3ds because they are fundamentally different concepts: one is a container, and the other is the content.
If you have found yourself with a Nintendo 3DS game file ending in .7z and are unsure how to load it onto your emulator or flashcart, you are not alone. This is one of the most common points of confusion for retro gaming enthusiasts.
A .3ds file from a .7z download.
A raw game ROM format used by emulators or a 3D mesh format used in graphics.