Gamefront has a preliminary screenshot of the recently announced Final Fantasy IV for GBA, which showcases a marginal improvement in graphics (including character portraits in dialogue windows) as was present in FFI&II: Dawn of Souls earlier this year. The scan is from the same issue which had the pictures of Children of Mana and Seiken Densetsu 4, and Gamefront mentions that FFIV will be appearing as part of a "Final Fantasy Projects" on Game Boy, presumably indicating that more such ports are on the way in the future.
FF3 was announced for DS and at the moment has vanished into obscurity, so there's little to no chance it'll come to GBA if it can't even make DS. It was once announced for Wonderswan, but that never happened either.