Can I play GBA Games on Nintendo 3DS?
N3DS does not have a GBA Slot like the NDS and DS-Lite, so You can not play Gameboy Advance games from cartridges, but You CAN play them with an emulator!
Gameboy Color and GAMEBOY classic games can be downloaded from 3DS Store and played using Virtual Console like on Wii, but there are no plans to let users download GBA, SNES, Sega Genesis or N64 games, so it is up to hackers and homebrew programmers to unlock 3DS so that it can play all the old games You already own (or roms You just downloaded from the net) on Nintendo 3DS with emulators.
Buy Nintendo 3DS SuperCard DStwo with a built-in GBA and SNES rom emulator! »
Nintendo 3DS Hardware - R4 3DS