Its really up to you actually. For me, I use only my index and middle fingers, and alternate the fingers like I--M-I/M--I-M/I--M-I and so forth for faster songs like The Trooper or Run to the Hills. For slower songs, I usually accent the first note and start with the middle finger. I don't alternate for slower songs (like Hallowed be thy name). So its M--I-M/M--I-M/M--I-M/M--I-M and so forth. By the way, a gallop isn't the same as a triplet, just to clarify. A gallop is a eight-note followed by 2 sixteenth notes. A triplet is 3 notes of equal measure. I'm not musically trained, so I'm not sure if im using the right words. Heh.