On The Big Bang Theory

As an adult, Sheldon Cooper has a ritualistic knock that he uses whenever he's knocking on somebody's door. As established on The Big Bang Theory, Sheldon knocks three times then says the person's name and he typically performs this three times.

On Young Sheldon

Sheldon has not yet fully developed his later knocking pattern. In its embryonic form, Sheldon still knocks three times and calls out the person's name, but he doesn't yet do this in triplicate.

Sheldon's knock was first seen in the episode "A Patch, a Modem, and a Zantac", when Sheldon visited Meemaw's house to ask for money to buy a computer. He knocked three times and said Meemaw, but he only did this once. 

The knocking ritual was extended to two rounds in "An Eagle Feather, a String Bean, and an Eskimo", as Sheldon visited Meemaw to show off the uniform for his new school. Sheldon stuck with two rounds when he visited Meemaw in "Summer Sausage, a Pocket Poncho, and Tony Danza" to ask her about her date with Dr. John Sturgis.

When Sheldon woke Georgie up for help with his paper route in "A High-Pitched Buzz and Training Wheels", he performed two rounds of his knock on Georgie's headboard. Sheldon again stuck with two rounds when he knocked on Georgie's bedroom door to prank him in "A Stunted Childhood and a Can of Fancy Mixed Nuts".