The Rule of Three is common in many fields, from comedy to debate to music to speechifying or story structure. Recognition of the importance of groups of three and how they impact the mind goes back at least to Aristotle, and was valued by civilizations that followed, from the Romans onward.
One such field is, of course, baseball —three strikes, three outs, 3x3 players, 3x3 innings and distances from base to base and home to mound in multiples of three. The use of so many threes helps make the game perfect.
Back in 2009 Dave Fleming proposed taking the Rule of Three one major step further and applying it to a pitching rotation.