I was a big fan of Joyce DeWitt on Three's Company, but a Jack and Janet spin off just wouldn't have worked for several reasons. First of all, over the course of the series the relationship between Jack and the female roommates moved strongly in the direction of a brother-sister relationship instead of the romantic one. To see these pseudo-siblings married might have seemed as out of place as a Brady Bunch spin off called "Greg Loves Marcia".
The second reason it would have failed is that "Three's Company" broke some social TV taboos in its day, so the successor should break some in its own day. Back in the 80s, the controversial trend was to dismiss the concept of marriage with the idea that you didn't need a contract from the government in order to be in a committed loving relationship (yet oddly enough the controversial trend in our current decade is the opposite belief) so having Jack shack up with a woman was the next logical step. Jack living with Janet, however, would not have made sense because both characters had previously expressed value in the concept of marriage and we've already seen them living together for the past 7 years. What would we gain, especially when her parents already like Jack! A third reason it wouldn't have worked is that the entire franchise was based upon the British "Man About the House" franchise. I understand the value in copying the core concept, but I don't know why the producers continued mirroring that franchise. (Legal reasons perhaps?) At any rate, "Three's A Crowd" was designed after "Robin's Nest" and trying to force Janet and her family into those roles would have been awkward. The bitter relationship between the parents of Jack's girlfriend was key to the reason behind their living together and it was also the source of a lot of comedy with the un-Father-in-Law. (It's odd. Vicki wanted this arrangement so that they were living together because they wanted to live together instead of being forced to live together. Apparently splitting up a relationship where two people share the same living environment, property, bills, and possibly kids is only difficult if that couple is married) We already met Janet's parents and they seemed fairly contented with each other... and fairly boring too.
I also think the producers wanted to get lots of fresh blood into the mix. If the female lead was Janet the name of the series might as well have been called "Three's Company Lite". (Though the series "Angel" did show that you can create a new series with a cast comprised completely from a subset of the cast of another show yet still have it feel like its own show) But all my arguments are a moot point considering that the series did fail.
