If you like to play only NDS games on your Nintendo 3DS you may use most of NDS flash carts.
Sky3DS+ => the best choice for Nintendo 3DS when it comes to 3DS game compatibility. But it can´t handle any other formats like nds or gba, emulators or any retro games.
SuperCard DS Two Plus => this is the best choice for Nintendo NDS Lite or Nintendo 3DS, for retro, emulators, multimedia fies, nds games and 3DS games with some lower compatibility than Sky3DS+. It have very advanced hardware, better than any other flash carts, it's fast.
EZ Flash 4 => is the best choice for those who have the GBA. It works on the fat NDS in Slot-2 as well.