Let's look at three possible origins, expressions, or foundations for this idea that naturally reinforces a copycat mentality in reporting histories, tragedies, and breaking news:
Avery's Rule of Three
"Trouble strikes in series of threes, but when working around the house the next job after a series of three is not the fourth job -- it's the start of a brand new series of three."
The Rule of Three in Writing
The rule of three is a principle in English writing that suggests that things that come in threes are inherently funnier, more satisfying, or more effective than other numbers of things. From slogans ("Go, fight, win!") to films, many things are structured in threes. There were three musketeers, three little pigs, three billy goats Gruff, Goldilocks and the three bears, and Three Stooges.
A series of three is often used to create a progression in which the tension is created, then built up, and finally released. Similarly, adjectives are often grouped together in threes in order to emphasize an idea.
The Rule of Three in Wicca
The Rule of Three (also Three-fold Law or Law of Return) is a religious tenet held by some Wiccans. It states that whatever energy a person puts out into the world, be it positive or negative, will be returned to that person three times. Some subscribe to a variant of this law in which return is not necessarily threefold.
According to John Coughlin the Law posits "a literal reward or punishment tied to one's actions, particularly when it comes to working magic." The law is not a universal article of faith among Wiccans, and "there are many Wiccans, experienced and new alike, who view the Law of Return as an over-elaboration on the Wiccan Rede." Some Wiccans believe that it is a modern innovation based on Christian morality.
The Rule of Three has been compared by Karl Lembke to other ethics of reciprocity, such as the concept of karma in Dharmic religions and the Christian edict, "Do unto others as you would have them do unto you" (Matthew 7:12), also called the "Golden Rule."
The Rule of Three has a possible prototype in a piece of Wiccan liturgy which first appeared in print in Gerald Gardner's 1949 novel High Magic's Aid:
'Thou hast obeyed the Law. But mark well, when thou receivest good, so equally art bound to return good threefold.' (For this is the joke in witchcraft, the witch knows, though the initiate does not, that she will get three times what she gave, so she does not strike hard.)
The first published reference to the Rule of Three as a general ethical principle may be from Raymond Buckland, in a 1968 article for Beyond magazine. The Rule of Three later features within a poem of 26 couplets titled "Rede of the Wiccae," published by Gwen Thompson in 1975 in Green Egg vol. 8, no. 69 and attributed to her grandmother Adriana Porter.
Trilogies of Death
BTW, this sense of a trilogy among death is found in various popular cultures and can be demonstrated by noting the following three collected works:
1) Three Deaths: A Tale (Russian: Три смерти, Tri smerti) is a short story by Leo Tolstoy first published in 1859. It narrates the deaths of three subjects: a noblewoman, a coachman and a tree.
2) Death Times Three is a collection of Nero Wolfe novellas by Rex Stout, published posthumously by Bantam Books in 1985. The book contains three stories, one never before published:
"Bitter End," first printed in the November 1940 issue of The American Magazine, and collected in the limited-edition volume Corsage: A Bouquet of Rex Stout (1977). The story is a re-working of Stout's Tecumseh Fox story Bad for Business.
"Frame-Up for Murder," an expanded rewrite of the 1958 novella "Murder Is No Joke" that was serialized in three issues of The Saturday Evening Post (June 21, June 28 and July 5, 1958) but never published in book form.
"Assault on a Brownstone," an early draft of the 1961 novella "Counterfeit for Murder".
3) Curse, Death & Spirit is a 1992 three-part anthology of Japanese ghost stories from popular director Hideo Nakata (Ringu, The Ring 2).
The movie consist of three storylines.
The Cursed Doll: Satomi is being called by someone in her dreams. Following this voice, the voice seemed to be coming from a doll in which she realize that there is a spirit of her dead sister inside of it.
The Spirit of the Dead: A woman takes her son camping after her husband's death but was disrupted when a ghost of a woman thinks that the boy is her child.
The Haunted Inn: A spirit forces three girls who are on a vacation in a traditional Japanese inn, in which a tragedy had occurred a long time ago, to repeat the events of the day.
JFK and Threes
Writings of a "twilight language" nature on the JFK assassination often mention the first Masonic Temple site in Texas, Dealey Plaza, as the location of the shootings. When they go on and note that the killing of the President (the symbolic "King") occurred near the banks of the Trinity River, next to the Triple Overpass, lined up on a map on the 33 degree parallel with the Trinity Site in New Mexico, therefore, these may also be an extension of the "death comes in threes" literature.
Material on the last point is extensive within the triple-decker works in conspiracy, synchromystic, and assassination thoughts, which are often remarkably scholarly in nature.