Humor uses threes as the completion to a joke. "The Rule of Three" is part of the formula. Many stories have you heard start out, "There was a priest, a minister and a rabbi … ." Three characters seems to work well when you are populating your own stories to make them funnier.
A reading from the Book of Armaments, Chapter 4, Verses 16 to 20:
Then did he raise on high the Holy Hand Grenade of Antioch, saying, “Bless this, O Lord, that with it thou mayst blow thine enemies to tiny bits, in thy mercy.” And the people did rejoice and did feast upon the lambs and toads and tree-sloths and fruit-bats and orangutans and breakfast cereals …
Now did the Lord say, “First thou pullest the Holy Pin. Then thou must count to three. Three shall be the number of the counting and the number of the counting shall be three. Four shalt thou not count, neither shalt thou count two, excepting that thou then proceedeth to three. Five is right out.
Once the number three, being the number of the counting, be reached, then lobbest thou the Holy Hand Grenade in the direction of thine foe, who, being naughty in my sight, shall snuff it.”