Three's Company did better in their series but Man About The House did better in spinoffs.

Three's Company(1977-1984 174 episodes)
Man About The House(1973-1976 39 episodes)

The Ropers(1979-1980)
George & Mildred(1976-1979)

Three's A Crowd(1984-1985)
Robin's Nest(1977-1981)

MATH was pretty successful throughout its entire run, while TC petered out during its last season, IMO. The only reason MATH went off the air was because they felt it was time to end the show.

One thing I find interesting about it though is the fact that George & Mildred was even more successful than Three's Company. Both G&M and MATH had movies made based on them.