That song is called "Opening Theme" in both FF3j OST and FFIII DS OST. It's a great song, and it plays in FFs 1, 3, 4, 5, 6, 7, 8, 9 and 12, and also FFVII AC. Many call it the Prologue (only FFIV OST calls it that, though), but since it plays during the ending in several FFs, I prefer to call it "Final Fantasy", which is what the song has been called several times lately. There was some more discussion about the song on this thread (just don't get this song confused with the Prelude! ), and according to this post of mine, there is a midi version of it on VGMusic.