I've always used just two fingers, the same as Harris. Two important things to remember, One is to really feel those triplets and practice them as fast as you can possibly go for as long as you can possibly go, sooner or later you will improve or give up. Second, his technique of plucking involves a feel more akin to striking from the front of the string to the back instead of plucking up from the bottom of the string to the top. Different actions that will net different speeds.