isn't there a hack for Action Replay DSi to use it as a flashcard? So I was thinking, maybe I could use that to run the ds profile exploit (to install cia) Does anyone know if that would work?
If you have the version that runs homebrew through the microSD card, yes. However, those aren't made anymore, and I don't think you'll find a 3DS compatible one with a microSD card slot.
Some older action reply DSI's can't be updated to use with the 3DS. I own one of these. I also use an R4 SDHC Dual Core 2014 along with ROP multiboot because it installs the mset hack a lot quicker.