A class 2, probably not.does class 2 work?
if so, why would you want to waste money on a higher class?
Slow read speeds have been used as a way to detect flashcarts, that is why you want the high class rating. Example of this is the AP in some DS roms, a too slow speed and it causes the game to blank out giving you a solid color screen. The minimum class speeds for those games are a class 4 or higher on a flashcart that can handle that type of AP. With a 3DS, you will have to get something well higher than a class 4.
This is what is called a buffer underflow, the system is not reading data from the cart fast enough. AP can be set up to just halt the system if this ever happens.