Interesting information about this show
I just did a bit of research about this since, after seeing some of the episodes on Antenna TV, I was curious as to why it vanished so quickly. According to news articles of the time, this show finished higher in the ratings than "Miami Vice" during the 1984-85 season. What's more, it had about the same popularity of "Who's the Boss." Interesting, when you consider how long those other two survived. Also, "Three's a Crowd" began it's run with decent ratings (one of ABC's few modest successes that season), yet after showing initial promise (September-October 1984) was already considered for the chopping-block within a few months (February-March 1985). Also of note is that plans were underway, in the spring of 1985, for a second season in which Stuart Pankin would join the cast as the next door neighbor. Most intriguing, Season Two was to begin with the leads getting married (see "The Philadelphia Inquirer" 10 April 1985 p.D11 for these last two points). Moreover, after the show was announced as cancelled in early May, 1985, it was shown in reruns throughout the summer. I located several articles that list it as finishing in the top ten rated programs for the week, even as late as the early part of September. On the flip-side, ABC was really struggling at that time and John Ritter was making a lot of money ($100,000 per episode). Still, when the show finished taping in February, 1985, plans were to begin work on it again that August. I suppose the rumor (which I was unable to verify via my search) of John Ritter turning-down ABC's offer of a half-season renewal is the reason it did not last. Regardless, while it wasn't necessarily a good show, it seems that it may have perhaps been cancelled prematurely.
By the way, most of the information I gleaned was from the "Boston Globe," "New York Times," "Philadelphia Inquirer," and "Providence Journal" as well as the Canadian publications "The Gazette" (Montreal) and "The Globe and Mail" (Toronto).