Mythology
From the Three Graces to the Three Fates to the Three Furies,

"This is what I call the American idea, a government of the people, by the people, and for the people - a government of the principles of eternal justice, the unchanging law of God."
Human beings use threes to conceptualize everything. Three data
points are enough to describe an entire set. Take time for example. We
break it into before, during, and after. Look at the basic structure of
color.