Here's one for the résumé: ''Starred in Robert Redford's first movie ménage à trois.'' Actresses Betsy Brantley and Lise Cutter earned that distinctive credit in Havana, the homage to Casablancathat teams Redford with steamy Swede Lena Olin. Olin may be the star, but the most sizzling sequence in the film belongs to Brantley and Cutter as Diane and Patty, two straitlaced Americanitas looking for a wild time in the Cuban capital. And that they get: The frisky three-way love scene, driven by a rumba beat, is intercut with shots of Cuban government thugs murdering rebels. ''Redford was a virgin before us, as far as ménage à trois,'' says Cutter, 31. ''I remember reading the script and thinking, 'Robert Redford and two women? Never happen.' We broke him in real slow.'' Redford and director Sydney Pollack, working together for the seventh time, ''were amazingly gentle and quiet,'' during the half-day shoot of the scene, says Brantley, 35, who is married to Steven Soderbergh, the director of sex, lies, and videotape, 1989's Cannes Film Festival winner. ''It just took away all of the tension.''