This year, for the first time, the Public Broadcasting Service has convinced many of its stations to carry the same two hours of programming nationwide on at least three nights of the week. If the plan works, it could be extended during the season to other nights.

Among the new imported and domestic PBS offerings this season:

"Camera Three," for 25 years a cultural institution on the CBS television network, moves to public TV, where 40 half-hour shows will include 24 from the CBS vaults and 16 new productions for PBS.

"Connections: An Alternative View of Change" is a 10-part technological history of humanity investigating innovations that changed or at least nudged the world forevermore.

"Musical Comedy Tonight," a 90-minute special produced by Sylvia Fine Kaye (Mrs. Danny), stars Ethel Merman, Carol Burnett, Rock Hudson, Agnes DeMille and others in a tribute to the American musical theater (Monday, Oct. 1 on most stations).

"Song by Song," produced by Ned ("We Interrupt This Week") Sherrin, will offer musically illustrated salutes to such renowned pop-music composers and lyricists as Alan Jay Lerner, E. Y. Harburg and Lorenz Hart. Participating entertainers include Lena Horne, Elaine Stritch and Georgia Brown. (Premieres Oct. 22).

"Paul Robeson" stars James Earl Jones as the legendary and controversial singer and actor, in a one-man show produced for television by WETA in Washington. (Oct. 8).

"Masterpiece Theatre" returns with multi-part productions of H. E. Bate's "Love for Lydia" and further adventures of "The Duchess of Duke Street."

"Great Performances" boasts a lineup highlighted by TV adaptations of three stories by John Cheever; Liv Ullmann in Jean Cocteau's one-character play "The Human Voice"; and David Mamet's off-Broadway hit, "A Life in the Theater." From Vienna's State Opera House will come Franco Zeffirelli's new staging of Bizet's "Carmen" and from Bayreuth, an authentic German production of Wagner's "Tannhauser."

"The Shakespeare Plays" will continue with six new productions and three repeats. The new ones: "Twelfth Night," "The Tempest," "Henry IV, Part I," "Henry IV, Part II," "Henry V," and "Hamlet."