*"Ground Rules" premiered September 13, 1977, on the ABC television network as the first episode of season 2, while "Double Trouble" aired March 24, 1981, and was the 19th episode of season 5.
*Chris Mann's comprehensive 1998 book covering the series, Come and Knock on Our Door, is out of print but, as we say during the episode, is a must-read for fans of Three's Company.
*The book mentions the no-sex-among-the-roomates edict the show established but also points out how often Jack struck out with all girls, which made the show surprisingly chaste. John Ritter jokes that Jack Tripper got some all the time during the commercial breaks, or else he never would have been able to stand living with two single females.
*Norman Fell and Audra Lindley left the show for the spinoff The Ropers, but it wasn't exactly their idea, and in fact Fell extracted a promise that if the series didn't last a year, they could return to the flagship. Unfortunately for him, it lasted one short "season" and one "full"--28 total episodes--just long enough for the Ropers to be left out.
*The British show that served as the inspiration for Three's Company, Man About the House, was a big success that led to multiple spinoffs, movies, etc. It also spawned a spinoff of its own landlord characters--George and Martha.
*Norman Fell's habit of breaking the fourth wall by grinning directly at the camera made him the only character who got away with that. Longtime show director Dave Powers was strict about not doing that when he joined the series, and he tells Mann that he asked if he should let Roper continue that gimmick and was told that the audience had come to expect it.
*Don Knotts was a nervous wreck when he joined the show as Ralph Furley because he was unaccustomed to the 3-camera process before a live audience. He had stage fright early in his career and never conquered it.
*Suzanne Somers, embroiled in disputes with the producers, threatened to sue John Ritter and Joyce Dewitt at one point, and a huge rift developed. By the end of the fifth season, she was literally phoning it in, as Chrissy had "left to take care of her mother." She shot her scenes on a separate set, apart from the rest of the cast, and would then fly to Vegas to perform her ongoing stage show.
*21-year-old Jenilee Harrison, former Rams cheerleader, joined in season 5 as Chrissy's farm-raised cousin. In season 6, Harrison was phased out as Priscilla Barnes' Terri Alden moved into the apartment while Cindy went to UCLA.
*The classic theme song was written by Joe Raposo with producer Don Nicholl co-writing the lyrics. (Not that) Ray Charles and Julie Rinker are the singers, though the cast briefly gave it a go (It didn't work). Interesting tidbit: In the original opening sequence, Jack is so flummoxed by a beautiful woman passing by that he falls off his bike. The woman is Somers in a brunette wig.