Do Nintendo ds shop cards work with 3ds?

Yes they do work but I wouldn't recommend using them or else Nintendo will brick your system.

Currently, there are many R4 3DS cards which can on 3DS console, such as R4i SDHC 3DS, r4i gold 3ds, acekard 2i and so on. But all of them can only run DSi games, no r4 3ds card can run 3DS games.