Depends on the N3DS.
Firmware up to 9.2: Gateway 3DS is your best choice, but actually you don't even need a flashcard, there are several custom firmwares out there.
Firmware above 9.2: Sky3DS + mGBA emulator via one of smealum's several hax
And to play DS games, you need a DS flashcart. No other way. Best one is DSTWO