Relatively Speaking, a comedy made up of one-act plays by Ethan Coen, Woody Allen and Elaine May, began previews September 20 and opens October 20 on Broadway. Frequent Coen Brothers collaborator John Turturro directs the cast which includes Julie Kavner. Each of the three one-acts follows a different branch of a single family tree. Timothy J. DeBaets represents Edward Walson, the co-producer of Relatively Speaking. (Mr. Walson is also an independent film producer (City Island) and cable television owner, he and his family own Service Electric Cable TV systems).