Cable-TV viewers in about 3 million homes that receive the 24-hour Cable News Network will be able to see a three-way debate among Jimmy Carter, Ronald Reagan , and John Anderson Tuesday night. CNN has hired Constitution Hall in Washington, and Mr. Anderson has agreed to appear to answer the same questions the two other candidates will be asked at the League of Women Voters debate in Cleveland. After the opening question, CNN will interrupt with Mr. Anderson's being asked the same question by Daniel Schorr, a former CBS newsman now with CNN. The rest of the debate will be presented by a tape delay to give Mr. Anderson time to take part. CNN president Reese Schonfeld told Monitor TV critic Arthur Unger: "We are setting our own ground rules. Eventually the FCC may have to rule on the fairness of our action under the equal-time regulation, but this is the only way we belive the undecided votor is going to be able to hear the issues debated by all three leading candidates."