So far, I have tried to show how a story should contain three parts: the trigger, the meat of the story, and the climax/resolution. I have also tried to show that during the meat of a story there should be three obstacles: the parable, the story and the tension builder that leads directly to the climax and resolution.
There is another way in which three is an important consideration for any story. That has to do with characters, and specifically with primary characters. One may be the main character, but most often (though not always) there will be three primary characters of which the main character will be one.
To begin, let me say that it is very difficult to write a one primary character story, even in first person. It has been done, but it is very difficult to maintain interest for a long piece. Why? No relationships.
Remember: The primary purpose in telling a story is to get the audience to pay attention (the reader to keep reading) right to the very end.
Two is better because then you can at least have a protagonist and an antagonist. True, the relationship will be singular and strictly defined and as such it might work best in a short story; but at least with two primary characters there is some room to play off the relationship between the two. Even here, though, for a longer piece there are strict limits on what is possible.
Thus authors have often given their main character a third person to be, at the very least, a foil for their main character. Sherlock Holmes had his Doc. Watson as well as his antagonist, Doctor Moriarity. Tom Sawyer had his Huck Finn as well as his antagonist Injun Joe. Don Quixote had his Sancho Panza, and his Dulcinea. He did not really have an antagonist outside of his own mind. That being the case, a love interest is a really good choice.
Three is the optimal number. Consider Luke and Leah (Star Wars) fighting the rebellion without Han Solo to save them. Or for that matter, consider the young Jedi Luke going up against the Evil Emperor without Darth Vader to get in the way. Consider Frodo and Sam (Lord of the Rings) struggling to get to Mordor without the added dimension of Gollum. Or Harry and Ron (Harry Potter) without the added dimension of Hermione. Try and picture any of these stories without that third wheel. Could the story have worked? Probably, but it would not have been nearly as good.
It is inevitably the third wheel that multiplies the relationships and gives depth to a whole work. This is true even if the third primary character is little more than a device or plot catalyst such as in the case of many fairy tales. Without Repunzel, though, why should the Prince and the Witch come to blows? Without the Prince, why should Cinderella hope and her Stepmother worry? Thus we have the wolf, Red Riding Hood and “Grandma, what big eyes you have.” Three works!
Four or more primary characters is certainly possible if you don’t mind coming off like a Russian novel and if you can keep all of the relationships straight. Now, you might say, “Wait a minute. I can think of plenty of stories of four or more characters.” But how many of those characters are primary and how many are secondary?
Doc Watson and Sancho Panza might both be considered supporting actors, but in some ways they are as integral to their stories as Darth Vader is when he stands between Luke and the Emperor. In that sense, the third wheels may be supporting but cannot be considered “secondary” Let’s face it, antagonists, nearly always primary characters, often have very little to do with the action. Sauron in Lord of the Rings is merely an eye. But as a figure looming in the distance, that eye is a character integral to the story around which so much of the action takes place, and as the primary catalyst of the whole story, one can only consider Sauron as a primary character. The others? The Hobbit and the King…
Then there were 4 as in Peter, Susan, Edmund and Lucy in C. S. Lewis’ classic tale of Narnia. Yet, I would argue that in a very real sense, though one betrays them, the children act almost as a character unto themselves. In other words, I feel Lewis might just as well have named the book “The Lion, the Witch and the Children.”
Of course, a story can be written with any number of primary characters, but three works great. It allows for a variety of relationships and sufficient complexity to carry a story a long way. Four or more, on the other hand, starts to get too complex, not only for the author, but also for a reader. It is something to consider.