After a quick online search, here is a list I found.... (I have not seen any of these)
"All That Jazz" (1979) Bob Fosse (Roy Sheider, Jessica Lange, Leland Palmer, Ann Reinking, John Lithgow, Ben Vereen, Sandahl Bergman, Wallace Shawn)
This musical semi-autobiography of drinking, smoking, pill-popping, womanizing choreographer Bob Fosse spends a large fraction of its time dealing with his constant cheating and the effects it has on the people who love him. More cheating than polyam, more polyam than polyfi, but well done and doesn't pull any punches.
"Anna and the King" (1999) Andy Tennant (Jody Foster, Chow Yun-fat)
Based on the story of Anna Leonowen.
"Big Top Pee-wee" (1988) Randal Kleiser (Pee-wee Herman, Kris Kristofferson, Valeria Golino)
A subplot has Pee-wee's former fiancee Winnie taking up with the 3 or 4 Piccolapupula brothers. Boy, does she look happy!
"Bob & Carol & Ted & Alice" (1969) Paul Mazursky (Robert Culp, Natalie Wood, Elliot Gould, Dyan Cannon)
More a wife-swapping tale than any model for a healthy poly relationship. Still, widely known, and helped raise poly questions in many people's minds when it first came out.
"Butch Cassidy and the Sundance Kid" (1969) George Roy Hill (Paul Newman, Robert Redford, Katharine Ross)
"Has a very active poly flavor to it. While it was never stated as such, both men seemed to have a very active relationship with the woman." - Gene Wolf
"Cafe au Lait" French film about a woman who becomes pregnant by one of her lovers. The three of them wind up forming a family of sorts to raise the child.
"Carrington" (1995) Christopher Hampton (Emma Thompson, Jonathan Pryce [best actor, Cannes])
"The true story of a bunch of writers and artists in England in the 20's, 'The Bloomsbury group', of which perhaps Virginia Woolf is the most well-known outside literary circles but she does not appear in the film. Jonathan Pryce plays a gay man; Emma Thompson is straight but in love with him; eventually she marries another, bisexual man and the three of them live together for some time. Problems occur when (1) Thompson's first fiancee has difficulties giving her up; (2) Thompson has a secret affair with the bisexual man's closest friend; (3) the bisexual man openly introduces another woman into the arrangement, which Thompson doesn't mind but Pryce does; (4) Pryce falls ill." - (anonymous)
"A Change of Seasons" (1980) Richard Lang (Shirley Maclaine, Anthony Hopkins, Bo Derek, Mary Beth Hurt)
A surprisingly good performance from Bo Derek, and a superb one from Shirley Maclaine, help elevate this movie above the norm. Maclaine's professor husband (Hopkins) has an affair with Derek; she then has an affair of her own, and the four decide to go on a holiday together to see whether they can work something out ...
"Enemies: A Love Story" (1989) Paul Mazursky (Anjelica Huston, Lena Olin)
From a story by Isaac Bashevis Singer. A man with three female partners gets very different things from each relationship.
"An interesting depiction of what I call 'squeaky wheel poly', where the person who yells the loudest about problems is the one who gets most of the attention" - Stef Jones
"French Twist" ["Gazon Maudit"] (1995) Josiane Balasko (Victoria Abril, Josiane Balasko, Alain Chabat)
About a woman who ends up with a husband and a wife. Very funny, with lots of realistic irrationality as they work through all the issues. There is a review at the Out magazine web site which includes the following quote: "Everybody dreams of having this, your house, your husband, your children, and your wife," Abril says with an impish laugh. "You would have no guilt, no clandestinity, no lies. What more could you ask for?" How about a sequel?
"The Hunger" (1983) Tony Scott (Susan Sarandon, David Bowie, Catherine Deneuve)
More bi than poly, and only a few moments of that (between Deneuve and Sarandon, and then at the very end Sarandon appears to have new partners of both genders).
"Lovin' Molly" (1974) Sidney Lumet (Beau Bridges, Blythe Danner, Anthony Perkins, Susan Sarandon)
Follows the lives of three men and the one woman they share from the time they are children until they are seniors. One man, who you never see, is the legal husband (and her first lover), the other two are best friends and her alternate lovers. Based on "Leaving Cheyenne" by Larry McMurtry.
"Micki + Maude" (1984) Blake Edwards
(Amy Irving, Dudley Moore, Ann Reinking, Wallace Shawn, Andre the Giant)
A man who wants a child badly ends up getting both wife and girlfriend pregnant.
"No Way Out" (1987) Roger Donaldson (Kevin Costner, Sean Young, Gene Hackman)
Costner gets involved with Young, who is already involved with his boss (Hackman). Bad things happen. She actively says that she is poly (before she is killed).
"Paint Your Wagon"(1969) Joshua Logan (Lee Marvin, Clint Eastwood, Jean Seberg, Ray Walston)
Lee Marvin and Clint Eastwood live in a triple relationship with Jean Seberg. The woman is sold off to the highest bidder (Marvin) by her Mormon husband, then makes up her mind that she is in love with both Marvin and Eastwood. Since her former husband had two wives, she reasons, why can't she have two husbands? There is absolutely no hint of sexual involvement between the two men.
"Rita, Sue and Bob too" (1986) Alan Clarke (Siobhan Finneran, Michelle Holmes, George Costigan)
A married man's affair with two younger women causes his wife to leave him. Can the three lovers transmute their sexual interest into something more lasting?
"She's Gotta Have It" (1986) Spike Lee
A story of a woman with 3 boyfriends who each are trying to get her to give up the other boyfriends.
"Splendor" (1999) Greg Araki (Johnathon Schaech, Matt Keeslar, Kathleen Robertson)
"an excellent movie. It is about a MFM triad, and covers the beginning of the poly relationship, and they have to decide, when the female becomes pregnant, how to keep the family together. The two males have to work together to show how much they really need the female and how much they love her. ... a good ending"
"The Unbearable Lightness Of Being" (1988) Philip Kaufman (Lena Olin, Daniel Day-Lewis, Juliette Binoche)
The womanizing Tomas falls in love with monogamous Tereza, but only his lover Sabina truly understands him. Meanwhile, Russian tanks roll into Prague. Highly recommended.
"Willie and Phil" (1980) Paul Mazursky (Michael Ontkean, Ray Sharkey, Margot Kidder, Natalie Wood)
About a trio polyamorous relationship.
Sex between a man and a woman can be absolutely wonderful - provided you get between the right man and the right woman. ~Woody Allen