THREE'S COMPANY had, according to Wikipedia, dropped to 33rd place by its eighth (1983-84) season. But how objective could the producers be to think the THREE'S A CROWD spin-off sequel could work? It was coldly unappealing in its shiny mid-'80s way, and even the title suggests an exclusionary rejection of the original show (i.e., everybody but Ritter).
Let's face it, though, THREE'S COMPANY was never the most sophisticated of material. And the behind-the-scenes people who were providing that material were presumably the same ones making the decisions about if, what, how, and when -- so could we be surprised that the spin-off tanked?
Of course, who knows how long THREE'S COMPANY could have continued had they not attempted the sequel?... Ninth seasons tend to see a creative upswing. I think it could have run 11 seasons, until 1987, before closing shop.
And I never even really liked it all that much!