3DS fladshcart: http://www.dsflashcart.com
Currently there is no flashcart can play 3ds games on the 3ds console, all the flashcart would only play ds games on the 3ds or 3ds xl. because 3ds haven't been hacked yet. If there is any flashcart says their flashcart can play 3ds games, it is a fake card because there is no card can do it.
But there are always some rumors about the 3ds hacking and real 3ds flashcarts that play 3ds games. Such as the first one Crown3ds card, this is the first flashcart that claimed hacked the 3ds and can play 3ds games. this news was released last year and since the first news released, they haven't released any more news, so this card has stopped updating.
The second flashcart about playing 3ds games is the ace3ds pro which was released a few months ago. this card is said to be playing 3ds games, but after it was released, we realize that it can only play one 3ds game, so actually it is not a real 3ds flashcart, it is just a real 3ds game card that attached with a normal ds flashcart.
The last one, today another new team named RealSC-team Released SUpercard dstwo pro which is a new card that play 3ds mode. they just released the news, until now, no official site, no videos, pics about it, so it is just another rumor, we can not believe it until they can provide more proof.
So currently i would recommend you just get a ds flashcart for your 3ds, and the best cards for you youe 3ds are r4i gold 3ds or supercard dstwo.