Why does Sheldon Cooper knock on doors three times in The Big Bang Theory? Played by Jim Parsons, Sheldon Cooper was so integral to The Big Bang Theory's success, the show opted not to continue without him, ending after 12 seasons. Known for his anal retentiveness and socially awkward personality, many of Sheldon's quirks and traits would become running gags. His claim over a certain spot on the couch, his drafting of a relationship agreement for Amy, and his demand to hear "Soft Kitty" during times of illness. Perhaps the most recognizable Sheldon-ism, however, is knocking on doors three times.
The triple-knock was first introduced in The Big Bang Theory season 2's "The Euclid Alternative" and quickly became a common feature that fans could copy as easily as any catchphrase. Parsons' character would knock, say the person's name, then repeat the process twice more. Sheldon continued to knock three times (especially on the door of next-door neighbor, Penny) all the way through to season 10, where he started to outgrow the habit. But where did it come from in the first place?
While The Big Bang Theory's creators claim Sheldon Cooper wasn't conceived with any specific neurological conditions, fans have consistently attributed the character with both Autism and Obsessive-Compulsive Disorder. Intentionally or otherwise, The Big Bang Theory certainly leads the audience toward that conclusion. These diagnoses would explain many of Sheldon's ritual behaviors, as well as his attention to detail and preference for routine, and would also account for the compulsion to knock three times on doors. To support this, The Big Bang Theory has shown how Sheldon feels uncomfortable when someone answers after one or two knocks and the ritual isn't complete. Though the knocking is almost certainly linked to Sheldon's neurological state, The Big Bang Theory has provided a more specific reason for the behavior.
In The Big Bang Theory season 10's "The Hot Tub Contamination," Sheldon and Amy are struggling to coexist after recently moving in together, with the former's constant demands grating on his new partner. Penny takes Sheldon away to talk, and they spend the day complaining about their respective relationships. During their time, Sheldon recounts a story from childhood that, supposedly, no one else knows. Returning home from college at the age of 13, Sheldon found his father having an affair with another woman. Ever since then, he's knocked three times to give the person time to "get their pants on."
Of course, Sheldon isn't just knocking repeatedly to avoid seeing people have sex. If Penny was in the middle of intimate relations when Sheldon came calling, she'd just ignore him and Sheldon would walk away thinking his neighbor was out. Instead his knocking has taken on the painful emotions from the breakdown of his family unit. Sheldon believes knocking thrice would've saved him from seeing his dad mid-rut and, therefore, spared him the sadness that followed. Carrying out the ritual on Penny's door, or Raj's door, or Amy's door could be Sheldon's way of trying to control the situation and ensuring nothing bad is waiting for him on the other side. This would explain why Sheldon doesn't use the knock throughout the entirety of season 1 (because he doesn't care about anyone enough to), and also why he eventually stops (he becomes more adept at personal relationships).
Some have argued that playing Sheldon's obsessive-compulsive traits for laughs is exploitative of The Big Bang Theory, while others suggest making light of such behaviors helps encourage discussion and lessen stigma. There's a case for both, but at least The Big Bang Theory provided a more meaningful explanation for why Sheldon is the one who knocks - three times.