the ez 4 is a gba flashcard that works on a ds using a passme or such card (or indeed a full nds flashcard like acekard) the 3in1 is a card designed to work alongside the nds flashcard.
the 3in1 uses the nds and nds flashcard to install and run gba games, its also a rumble pack and a memory expansion pack for the ds browser and some homebrew titles (snes emulator, linux and offline wikipedia spring to mind).
the ez4 is basically a standalone flashcard for gba / nds slot 2 so it needs its own sd card for its own roms etc, another point is that the 3in1 works with an sdhc (upto 32gb) card in your slot 1 flashcard, the ez4 if i remember only takes sd so is limited to 2gb.
we now know the Xbox One's game licensing policy was written from the ground up for companies. It's aggressively anti-consumer and anti-middle class, and it outright ignores underprivileged gamers. It's gross, despicable, greedy, pathetic, cowardly and out of touch with a growing global resentment for corporations. - Polygon 7th June 2013