Certain motifs and patterns appear consistently in storytelling, whether it be fairy tales or literary fiction. The repetition of "threes" is particularly useful in writing humorous scenes and dialogue. Consider the following first draft:
"Kim scowled as she filled out the patient intake form, distracted by the screams of a diaper-clad toddler who managed to void his bladder while assaulting the receptionist."
Applying the rule of three, compare it to this version:
"Kim tried to ignore the screams of a diaper-clad toddler, who flung a sippy cup at the receptionist, screeched in delight at hitting his target, and celebrated the occasion by urinating in a potted fern."
(The above event may or may not describe a recent visit to a walk-in clinic.)
Now consider the use of threes in dialogue. Here is version one:
"Your repeated insults of my home town are off-putting," Bobby said.
"Listen, the other day I bought a hot dog off a guy in a landfill," Jess replied.
Cute, but add in a third line to add some extra zest to the debate:
"It is a salvage yard, not a landfill," Bobby replied, "And they are all beef kosher."
Experts say the human brain is hard-wired to perceive storytelling a specific manner, hence the centuries-old adherence to a three act structure with specific plot points. Three indicates a completeness, whether narrative or dialogue. So if you are conveying that your friend has a slightly creepy crush on your mom, see how it flows if you tighten the description to just three sentences:
"Most of the interaction between Steve and my mom happens in the kitchen. Mom flits around in her gym spandex, microwaving Hot Pockets while Steve pretends he actually wants a snack. I avoid the scene by stashing twinkies under my bed for an after school pick-me-up."
Let's say you and Steve have a confrontation about his disturbing fondness for your mom:
"Man, Debbie says you never have anything nice to say," Steve said.
"Stop coming over all the time," I shouted, "She's forty years old for god's sake! It's creepy"
"Debbie says forty is the new thirty."
Give it a whirl. Who is to argue with the likes of Aristotle that the magic number is three?