New at this - was thinking of getting an Acekard 2i to play some DS/DSi game backups on my 3DS. Is it also possible to play GBA game backups on the 3DS with it?
No.
The only carts that play GBA games on anything other than a DS/Lite are the iSmart and DSTwo series. Of those, the DSTwo is the recommended one.
Yes. Pretty much every cart that works on the DSi works on the 3DS.