Triskaphobia

Triphobia or triophobia (from Greek/Latin tri, "three"), also known as triskaphobia, is the fear of. People may fear that three represents the limited number of warnings till they are penalized or resulted in other bad outcome, like 'three strikes, you're out'. Sufferers would often avoid situations where three is used, like getting to be part of three. Three can sometimes be considered an unlucky number because of the three different superstitions: the middle person in the photograph will die first;, meaning the third person to light a cigarette from the same match or lighter, which originated during World War I when a sniper might see the first light, takes aim on the second and fire on the third; and bad luck often come in threes.

Gabe Newell, director of video game development, is afraid of the number three. It is the reason why he doesn't develop third installment of the franchises, such as Half-Life 3.