Three provides rhythm and order.

We are conditioned to look for patterns when we process information, whether it is visual or auditory. Three is the smallest number that we will recognize as a pattern. In writing and in speech, it gives a satisfying sense of completion and rhythm. Consider "ready, willing and able", "Tom, Dick and Harry", or "signed, sealed, delivered." Three is natural to us. We eat three square meals, play "rock, paper, scissors" and get three strikes before we are out.