3DS Lib
The Nintendo 3DS Library.

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!
top GBA games 3DS playGameboy 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.

It may be that for emulating more demanding systems like N64, GBA, MAME and SNES You will have to use a 3DS Flash Card with a built in coprocessor similar to DSi SuperCard DStwo.

Buy R4 3DS DStwo GBA

Buy Nintendo 3DS SuperCard DStwo with a built-in GBA and SNES rom emulator! »

Resources:GBA on DSi DStwo 3DS

GBA on DSi
GameBoy Emulator
Nintendo 3DS Homebrew
3DS Emulator

GBA ROMs - Gameboy Advance Game Backups
MAME Roms - arcade game files
SNES Roms - Super NES 16-bit games for emulators

Nintendo 3DS Hardware - R4 3DS

R4 3DS Card - Flash Cards and microSD adapters for N3DS roms R4i DSi R4DS SDHC.
Download R4 DS Roms also known as NDS Games for R4i.

3DS Tech Specs

3DSemu
3DS Emulator Download
3DS PC Emulator