Nobody knows for sure why trilogies are so popular, but there are several good explanations: the three-part shape of a story, the example of The Lord of the Rings, the "rule of three", readers who want more of a world they love, and publishers who sell more books.
If you are an avid reader or you follow the film adaptations of books, you have probably noticed a certain trend. It is not the habit of splitting the final book of a series into two films, as with Harry Potter, The Hunger Games, and Twilight. It is the trilogy.
The term trilogy refers to three works of art, whether books, films, or another art form. The three parts are connected, and together they form one big work rather than three separate ones. They can be connected by a recurring theme alone, or by the same characters and setting. Trilogies and longer series are especially common in science fiction and fantasy.
But why is that? And why have so many authors, past and present, decided to tell their stories in three parts?
Why are trilogies so popular?
Unfortunately, there is no single answer to this question. Nor is there one to the question of why so many authors, editors, and publishers want to publish a story as a trilogy rather than as one book. There are several theories that try to explain this literary phenomenon, but none of them is the clear favorite.
A story has three parts
Maybe it all goes back to Aristotle? The philosopher wrote that a complete story has a beginning, a middle, and an end. Writers today often speak of a setup, a conflict, and a resolution. Could it be that trilogies follow this structure, with each book forming one of the three parts that make a good story?
The example of Tolkien
Or do writers and publishers simply still follow a pattern that J.R.R. Tolkien and The Lord of the Rings shaped? Tolkien saw his work as one long novel. It was published in three volumes in the 1950s, mainly because paper was scarce and expensive in post-war Britain. Still, the concept worked, and it sold. Most stories published as trilogies, or as longer series, are science fiction and fantasy, with crime novels following suit. So it does not sound too far-fetched to assume that trilogies are published in honor of the "father of modern fantasy".
The rule of three
A more playful explanation is the rule of three. "Third time lucky", people say, and the number three seems to play a big part in everyday life. We lift heavy things on the count of three, and we shake our fists three times before we play rock, paper, scissors. In many older video games, you had to hit a boss three times to beat it. Three seems to be a kind of magic number, and the trilogy may just be the literary version of the rule of three.
The reader's perspective
The original reason for trilogies may be hard to pin down, but there is no doubt that stories told in three or more parts can be very successful. The Hunger Games, Divergent, Stieg Larsson's Millennium trilogy, and even Fifty Shades of Grey were all first published in three parts, and all of them were hits. Some series went even further than the "required" three books, such as Harry Potter, A Song of Ice and Fire, and The Hitchhiker's Guide to the Galaxy, which was famously billed as "a trilogy in five parts".
Apart from the Millennium books and Fifty Shades, they all belong to roughly the same genres: fantasy and science fiction. That comes naturally, given the worlds in which these books take place. A new, fantastic world with many creatures, customs, and peoples can easily fill more than one book. Sometimes it needs more than one book to get the whole idea and beauty of such a world across to the reader. (If you are building one yourself, read about the basics of world building.) Such worlds also make us more curious than a story set in our own world or in a city we know all too well. There are so many new and exciting things to discover.
That is probably the main reason why readers feel so drawn to a trilogy or a series: we want to know more, and always more, about the fantastic worlds, the creatures in them, and of course the characters who have grown on us while reading. Opinions are split, though. For many readers, waiting for a sequel is a nuisance, and some fans wait for years. Yet publishing shows again and again that plenty of people are willing to wait for a new adventure with their favorite characters.
The publisher's and the author's perspective
And from the publisher's or the author's point of view? The most obvious answer is a financial one. A great story published in several books brings in more money. With every new part of a series, sales of the earlier parts, and especially of the first book, often rise. Merchandise can be produced, and film and TV adaptations of long series and trilogies can be big financial successes as well. A good story sells, and as long as it sells, publishers keep it going.
So whether or not you like the trend of telling stories in threes, it is here, and many iconic and highly recommended books come as trilogies. While you wait for the next book in a series, you can always pick up an exciting standalone novel in the meantime.