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. 

Primary colors in pigment (red, yellow, & blue) make up all colors. Color is also classified into primary, secondary, and tertiary. Look at our basic sentence structure to communicate. Subject, verb, object. In Japan the ordering is subject, object, verb. But they both utilize three components to communicate. The list goes on and on and on.