There is a hardware mod you can do to make card switches work in DS mode on the DSi and 3DS.
I believe it involved bridging those two pins together:
Note: Don't take my word for it, I'm only going off by memory there. Wait for other people to confirm or ask around before attempting this.
This is a DSi motherboard but the cartridge slot is the same on the 3DS and the same trick should work on both.