Following in the footsteps of mentor Norman Lear, producer Don Nicholl and his team adapted Thames Television's Man About the House into Three's Company, which launched as a mid-season replacement on ABC in 1977, and lasted 8 seasons, and almost as many cast changes.
Early scripts were adaptations from the original source, with House creators Johnnie Mortimer & Brian Cooke getting some credit. The plot centered on luckless bachelor Jack Tripper (John Ritter, ex-The Waltons) moving in with two attractive women, Janet Wood (Joyce DeWitt) & Chrissy Snow (Suzanne Somers), who'd just had a roommate move out. Landlord Stan Roper (Norman Fell, ex-Dan August) isn't too thrilled, so Jack has to lie and pretend to be gay to appease Roper, though the landlord's wife (Audra Lindley) might have other ideas.
The Ropers were the first ones to go, spun off into their own series after season 3, which led to sitcom legend Don Knotts (ex-The Andy Griffith Show) returning to television as new landlord Ralph Furley. Knotts had tried to carry his own show in the 70's, but failed, and it seems he's more at home as part of an ensemble, as he was with Andy Griffith, and, before that, Steve Allen. Somers left in a contract dispute, with Jennilee Harrison cast as Chrissy's sister, Cindy. However, Harrison left and joined the cast of Dallas, if memory serves me correctly, and thus Priscilla Barnes was brought on as the last of the roommates. Around season 2, Richard Kline joined the show as Larry Dallas, an upstairs neighbor who became Jack's best buddy and sidekick.
Janet was the only one who had a steady job during the course of the series, working in a flower shop. Jack would eventually get one as well, which would ultimately lead to the follow-up series, Three's A Crowd.
Edit: 11/20/13: YouTube has deleted the episode previously posted. In its place is the Season 4 open, which marks the beginning of the Don Knotts era:
TV Land holds the rights to the series at present (check listings), but the earliest seasons were the best.