The EZ-Flash 3-in-1 Expansion Pack is a device that goes into Slot 2 on the DS (the GBA cart slot) and provides GBA game capability, as well as Rumble and RAM expansion (for the web browser) features.
It was designed as an expansion for the EZ-V, but it can be used for playing GBA games with any slot 1 flashcart that has full DLDI support thanks to third party programs like GBALdr and the 3in1 Expansion Pack Tool. The rumble and RAM expansion features can be used with some (but not all) slot 1 flashcarts (the R4/M3 Simply and DSLink are best supported).
All GBA games that use EEPROM or Flash save types need to be patched to work with the 3-in-1.