As “Schoolhouse Rock” taught us, three is a magic number — especially in the movies.

Movies rely on the three-act structure. Romance films are often built on a love triangle. Trilogies, from “Back to the Future” to “The Lord of the Rings,” long have been the perfect vehicle for epic stories.

In movies, three is the right number of musketeers, amigos, caballeros, godfathers, stooges, men and a baby, days of the Condor and faces of Eve.

At the movies in 2017, certain trends also fell into threes. Here are a few of them:

• Movies that show the R-rated comedy fad may need to take a break: “The House,” “Father Figures,” “Rough Night.”

• Sequels that never should have gone past the script stage: “Kingsman: The Golden Circle,” “Despicable Me 3,” “Diary of a Wimpy Kid: The Long Haul.”

• Animated movies that are better than they had any right being: “Captain Underpants: The First Epic Movie,” “My Little Pony: The Movie,” “Cars 3.”

| Sony Amination / Columbia Hefty (voiced by Joe Mangianello), Brainy (voiced by Danny Pudi), Clumsy (background, voiced by Jack McBrayer), and Smurfette (voiced by Demi Lovato), from left, take a selfie, in a scene from "Smurfs: The Lost Village."

• Movies that cement the argument that Sony Pictures Animation should just give up: “Smurfs: The Lost Village,” “The Emoji Movie,” “The Star.”

• Movies where Woody Harrelson is better than anything going on around him: “The Glass Castle,” “LBJ,” “Wilson.”

• Movies where Matt Damon’s guy-next-door qualities don’t do him any favors: “Downsizing,” “Suburbicon,” “The Great Wall.”

This image released by Disney, Josh Gad, left, and Luke Evans appear in a scene from, "Beauty and the Beast." (Laurie Sparham/Disney via AP)

• Movies that show Josh Gad’s acting is best consumed in small doses: “Murder on the Orient Express,” “Marshall,” “Beauty and the Beast.”

• Movies where Alison Brie is great and you wish there were more scenes with her: “The Disaster Artist,” “The Post,” “The Little Hours.”

• Movies where Domhnall Gleeson makes your skin crawl: “American Made,” “mother!,” “Star Wars: The Last Jedi.”

• Movies where Michael Stuhlbarg makes the film better just by being in it for a few minutes: “Call Me by Your Name,” “The Shape of Water,” “The Post.”

This image released by A24 Films shows Tracy Letts, left, and Laurie Metcalf in a scene from "Lady Bird." (Merie Wallace/A24 via AP)

• Movies where Tracy Letts is a national treasure, even if he’s not writing his part: “Lady Bird,” “The Post,” “The Lovers.”

• Movies where great physical harm comes to Caleb Landry Jones: “Get Out,” “American Made,” “Three Billboards Outside Ebbing, Missouri.”

• Movies Dwayne Johnson will probably leave off his résumé: “The Fate of the Furious,” “Baywatch,” “Jumanji: Welcome to the Jungle.”

• Movies about ghosts that are as thoughtful as they are scary: “Personal Shopper,” “A Ghost Story,” “Coco.”

This image released by Roadside Attractions shows Valeria Hodos, from left, Samantha Barks and Lucy Brown in a scene from, "Bitter Harvest." (Roadside Attractions via AP)

• Movies that prove melodrama is a poor way to chronicle a genocide: “The Promise,” “Bitter Harvest,” “Viceroy’s House.”

• Movies where a famous writer discovers the downside of fame in the most annoying way possible: “Rebel in the Rye,” “Goodbye Christopher Robin,” “The Only Living Boy in New York.”

• Movies where women serve poisonous mushrooms to abusive men: “The Beguiled,” “Lady Macbeth,” “Phantom Thread.”

Nicola Dove | STX Films Gemma Arterton plays Catrin Cole, a fledgling screenwriter assigned to write uplifting propaganda films for the British war effort during World War II, in the romantic comedy-drama "Their Finest."

• Movies about the retreat at Dunkirk in World War II: “Dunkirk,” “Darkest Hour,” “Their Finest.”

• Movies with “Wonder” in the title that don’t involve Wonder Woman: “Wonder,” “Wonder Wheel,” “Wonderstruck.”

• Movies that prominently feature Wonder Woman: “Wonder Woman,” “Justice League,” “Professor Marston and the Wonder Women.”