A .3DS file is a raw, uncompressed dump of a Nintendo 3DS game cartridge. This is the standard format used by PC emulators like Citra.
Citra is the leading open-source emulator for 3DS games. To play 3DS ROMs on Citra:
To install CIA files, your 3DS must have Custom Firmware (CFW) via Boot9Strap and Luma3DS. Here is the standard workflow: 3ds roms cia
With the original Citra emulator shut down by Nintendo/Yuzu developers in early 2024, the community has risen again with open-source forks.
Note: These emulators generally do not play CIA files natively. You must convert your CIA to a .3ds format using a tool like 3dsconv or GodMode9. Note: These emulators generally do not play CIA
Key takeaway: If you are emulating on a PC, you want .3ds files. If you are playing on a real, hacked 3DS console, you want .cia files.
Many "essential" 3DS games have been ported to the Nintendo Switch legally: hacked 3DS console
Before CFW was perfected, users relied on flashcarts. These devices sit in the cartridge slot and read .3DS files off a microSD card. While still functional, CFW is generally superior today as it allows for CIA installation and online play (with risks).
Before searching for these files, users should understand the dangers of the "scene."