What I would have done with this show
Here's what I would have done if I controlled the universe, and as a result, controlled Three's a Crowd.
Jack and Janet get married in the final episode of Three's Company and buy a house. Mr. Furley somehow loses the apartment and they rent out a room to him. It's basically Jack and Janet trying to make things work as a married couple (dealing with their careers and such) while Mr. Furley lives there and more or less mooches off of them. It would be interesting to see Furley enter the dating scene and find jobs at his age while living with Jack and Janet. Furley would be the goofy guy while Jack would be more of the comedic straight man (which Ritter did more of later in his career).
I don't know, maybe that's a bad idea. I can't figure out how they would put Larry and maybe Chrissy in it too.