The most common scenario is that your .7z file already contains a .3ds or .cia file inside. You do not need a specialized "converter" for this; you need an extraction tool.
You usually need a .cia (CTR Importable Archive) to install via the FBI app.
You can use on your console to convert an extracted .3ds file into a .cia .
A refers to the process of extracting these archives and preparing them (repacking) into a format the console or emulator can read. Understanding the 7z to 3DS Workflow
Use the official 7-Zip or PeaZip to right-click the file and select "Extract Here".
Managing your Nintendo 3DS library often involves dealing with large compressed archives. If you have downloaded game files or backups, they likely arrived in a format—a high-compression archive that saves space but cannot be played directly on hardware or emulators like Citra.
Advanced users can use 3DSkit to extract, repack, and convert various Nintendo file formats directly from a command line. Key Differences Between File Types 3ds guide for my friends