Three's Company was based on the Thames series, Man About the House, created by Johnnie Mortimer & Brian Cooke. It outlasted House, which lasted just 3 years (1973-6), while Company lasted nearly a decade. Each series had two spinoffs, neither of which were exactly equal to its parents.
In the case of Company, the failure of The Ropers should've been a warning sign that trying a second spin-off, mirroring the House pattern, was not a good idea. Undaunted, ABC went ahead with Three's a Crowd for the 1984-5 season. That was the good news. The basic concept of the series was being tweaked, as Jack Tripper (John Ritter) now was a one-woman man at last, that one woman being Vicky Bradford (Mary Cadorette, who never landed another series), whose father (Robert Mandan, fresh from Soap) was now their landlord.
Unfortunately, ABC rolled the dice by putting Crowd opposite NBC's The A-Team. Ballgame's over, sucka. Crowd was cancelled after just 1 season. I get that they needed a tentpole at the front of the lineup to replace the 1-2 punch of Happy Days & Laverne & Shirley, which had ended their runs, but moving the more adult comedy of Crowd to the front of the line made no sense.
Ultimately, the demise of Crowd began a string of failures for Ritter, whose next ABC entry, Hooperman, Steven Bochco's attempt at a modern dramedy, was a critics' darling but a ratings failure. Ritter would only snap the string by moving out of primetime and on to kids' programming, as he was the original voice of PBS' Clifford the Big Red Dog, and that was before his last primetime series, 8 Simple Rules (For Dating My Teenage Daughter).
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No rating. Never saw the show.