In this post I mentioned the rule of threes. I probably explained it badly. Perhaps the following illustration will help.
One day a barber cut a teacher's hair. When he got done, the teacher asked, "What do I owe you?" The barber said, "You teachers put in long hours and get little pay or recognition. You educate our children and help to make the future brighter. Please, accept this haircut as a token of my thanks."
The next morning when he came to work, the barber found a dozen apples sitting on his doorstep.
That day the barber cut a police officer's hair. When he got done, the cop asked him, "What's the damages?" The barber said, "You know you police officers do so much for our community. You protect our lives and property. You preserve order and keep us safe. This haircut is on me."
The next morning when he came to work, he found a dozen donuts sitting on his doorstep.
That day the barber cut a preacher's hair. When the preacher said, "How much will that be?" The barber said, "You clergy do a lot for our world. You preach the Gospel. You baptize and marry and bury us. You comfort the bereaved and visit the sick. There's no charge for this haircut."
The next morning when he came to work the barber found a dozen preachers sitting on his doorstep.
It's an old joke, but it illustrates the Rule of Threes nicely. The first iteration sets up the premise. The second iteration sets the pattern. The third iteration breaks the pattern for comic effect.