Look for files ending in .3ds (for emulators) or .cia (for installing directly onto a 3DS console). Technical Considerations for Emulation
: Focuses on DLC and add-on content for 3DS games in the .cia format.
Assuming you have legally obtained a decrypted ROM (e.g., you dumped your own cartridge and decrypted it, or you are downloading a homebrew game), here is the standard workflow.