A mostly harmless spin-off (which I haven't seen in syndication in almost 10 years), this followed Jack Tripper to his new home - with a new cantankerous landlord who also happens to be the father of his live-in girlfriend. The jokes had grown old, and John Ritter started to look trapped in the Tripper persona. Not a show to be well remembered.
While Mary Cordette did an adequate job as Jack's love interest, I think it would have been much better for the show had, at the end of Three's Company, Jack and Janet realized there were deeper feelings for each other than had previously been recognized. The spin-off following them would have likely been more successful (and popular; I seem to recall that fans of the original show often clamored for a romance between the two characters who lasted the entire run of the show).