I thought Three's Company was rather funny. At least in the first three seasons. Then like most sitcoms, they had a tendency to get dumb and dumber.
Such as Happy Days: it started out as a cute sitcom, funny at times, good enough entertainment for the family. Then near the final season, The Fonz was wearing his leather jacket while water skiing, and performing the death defying stunt of jumping over a shark in the water.
You mentioned dumb shows from the 70s and 80s. But don't forget the 60s too; who can forget Gilligan's Island?