Today, I successfully modded it. First, I used screw drivers to take out the screws in both the EZ Flash 3-in-1 and the GBA game. I noticed that the 3-in-1 could not fit inside of the GBA cart because it has a plastic divider. I didnt really have any tool to take that out, so I just used the back part of the cartridge. I hotglued the back of the 3-in-1 to the cartridge, then I glued the top of the EZ 3-in-1 cartridge on top of it.
It now looks like a regular GBA game, but the top of the cartridge just has empty space. If you are going to attempt this, make sure not to put the hot glue too thick under the 3-in-1 because when it dries it will make the teeth of the 3-in-1 go too high from the cart so it won't fit in the GBA. Have fun!
Edit: Thanks, Jeffrey