As far as I know there is no way of editing 3DS game cartridges. In theory modifying at least "card2" games could be possible. How? No idea.I am totally new to this but I was wondering if there was a way to transfer a GBA hack file to a 3DS game cartridge. I know I would have to do some editing to my 3DS, but I just wanted to know the extent.
Thanks
https://gbatemp.net/threads/is-there-any-way-to-edit-contents-of-a-3ds-cartidge.555320/
If your goal is building anything like a "reproduction cartridge" to resemble a retail game as close as possible, the logical thing would be putting a GBA romhack on a GBA repro. If I remember correctly there are blank EEPROM cartridges and some bootlegs GBA carts can be overwritten.
It would not make sense to write own software on a 3DS game cartridge anyway as an unmodified 3DS will not start any unsigned software – defeating very much the purpose of such a reproduction… and I guess loading GBA games from cartridge would not work anyway – AGBFIRM was never built to support this (the only official GBA games for the 3DS are those from the ambassador program and those are download software for playing from SD like any eShop game).
To simply play a Game Boy Advance ROM(-hack) on a 2|3DS: Virtual Console injection using NSUI (New Super Ultimate Injector) is near to perfect.