The PassCard3 is a Nopass device which looks just like a DS game and goes in the DS game cart slot. It is required for flashcart use as it tells the DS, on startup, to enable DS games and files to run from the GBA slot.
I suggest getting a SuperKey if its cheaper as it is the same exact product with a different sticker but is usually cheaper.
Also if you want a flush fitting solution that is just as good if even better then the M3 miniSD then get a SuperCard. Get the SuperCard Lite if you have a DS Lite and a SuperCard miniSD if you have a Phat. Both products sit flush in the GBA slot and allow you to play all of your fav DS, GBA, and homebrew games and apps! Not to mention the SuperCard is much cheaper then the M3 products.
FifthE1ement