Guess who's knocking on John Ritter's door? Hint: The ''8 Simple Rules'' star is taking a Tripper down memory lane. In the ABC family-com's Jan. 28 episode, Paul Hennessy (Ritter) falls asleep while watching television and dreams that he and his family members are spoofing characters from ''Three's Company.'' (Instead of Jack-ing himself up, though, Ritter will do his best Mr. Roper; look for a cameo by Don Knotts as well.)
''Every so often, I'd feel an army of tiny little goose bumps marching up the spine to my neck and back behind my ears,'' marvels Ritter about the retro-shoot. ''I'd look at this living room with the yellow rug and the orange stuff, and I'm going 'This is exactly it.''' He was equally impressed by his ''Rules'' costars, who prepared by boning up on ''Company'' episodes. ''The kids took it so seriously,'' he notes. ''It was like they were studying for finals.'' No word yet if a ''Hearts Afire'' dream sequence is next.