The allure of the triangle

Returning to Aristotle for class today, I found myself focusing on his use of threes in defining rhetoric in its subjects, kinds, ends, etc. Why threes? What is it about the number itself? Why a rhetorical triangle? What makes three the right balance – two as divided, four as a multiple of two’s divisions, and upward too many? One doesn’t imply balance and is as encompassing as a circle, minding scope. Perhaps it was easy to draw.