The Rule of three is... one of those 'laws' about the Planes, about everything tending to happen in threes... or everything's composed of three parts, or there's always three choices, and so on and so forth.
— Morte, Torment
Sea Chimp: If white one with mouth truly is high priest, he prove it by fulfilling the ancient prophecies!
Max: I bet there are three of them.
Sea Chimp: He knows our ancient traditions!
—Sam & Max: Moai Better Blues
Who would cross the Bridge of Death must answer me these questions three!
— The Old Man from Scene 24, Monty Python and the Holy Grail
"Just the place for a Snark!" the Bellman cried,
As he landed his crew with care;
Supporting each man on the top of the tide
By a finger entwined in his hair.
"Just the place for a Snark! I have said it twice:
That alone should encourage the crew.
Just the place for a Snark! I have said it thrice:
What I tell you three times is true."
— Lewis Carroll, The Hunting Of The Snark.
Genies don't grant just two wishes, nobody talks about the two Musketeers, and you never hear anyone say "second time's the charm."
Stories are like Charlie's Angels! They should come in threes!
—Disco Stu, Simpsons Comics #175
Three is a sacred number. There are three Fates, three Furies, three Olympian sons of Kronos. It is a good strong number that stands against many dangers.
— Chiron, Percy Jackson and the Olympians
One time is happenstance, two times is coincidence, three times is intentional.
— An old adage.
They took my dressing room, my parking space, even my writers so I don't have a funny third item!
Amy: So that was the year of the slow invasion. When the Earth got cubed and the Doctor came to stay. It was also when we realized something the Shakri never understood. What 'cubed' actually means. The Power of Three.
— Doctor Who, "The Power Of Three"
What is it with threes? Eidolon controls three powers, Valkyrie controls three “warriors,” that Endbringer copied three capes. The are three Triumvirates. The first with Alexandria, Eidolon, and Legend. The new one with Legend, Chevalier, and Valkyrie. And one that was hinted at with Riley, Nilbog, and Valkyrie. Taylor had three different names; Skitter, Weaver, and Khepri. There are three entities in the story. I don’t know, maybe its a coincidence.
There was even a ‘three’ at the very beginning–Emma, Sophia, and Madison.