King Crimson has always been about experimentation and pushing boundaries. When the "double trio" first came together, with two drummers, two guitars, and two bass/Stick players, I expected total cacaphony, but was instead amazed by how tight everything sounded, which was the exact opposite. Everything fit, nothing felt excessive or superfluous.
Fripp is a genius in pretty much all aspects of music, including structure and arrangement. Are three drummers "necessary"? Probably not. Can it work and sound awesome? Hell, yes!