Not Three’s Company XXX was a huge hit for Hustler/X-Play last spring, and there have been many Three’s Company porn parodies over the years. Almost certainly the first was Sissy’s Hot Summer in 1979.
From the back cover: “This story is an X rated take off an old hit TV series about three roommates … Sissy, Janet, and Jack. Their cozy sexual threesome becomes endangered by eviction when they fall behind in the rent, so they must hustle the only thing they know how to do for money.”
Sharon Kane (billed as “Jennifer Walker”) played Sissy, while John Holmes appeared as “an eccentric playboy who thinks he’s Tarzan”. Candida Royalle had a non-sex role as a late-night movie show hostess reading fake commercials between sex scenes.