Barbara Walters, Walter Cronkite and John Chancellor - themselves - all left for Tel Aviv Thursday night to anchor network coverage of what one ABC News executive calls "the biggest Mideast story since Ramses IV," Egyptian President Anwar Sadat's visit to Israel and his speech to the Israeli parliament.

All three networks plan extensive coverage today and Sunday of the event, one which they helped to bring about. Heavy and splashy coverage of Sadat's offer to visit Israel on Monday night network newscasts is considered a key factor in Israeli Prime Minister Menachem Begin's decision to accept.

ABC's scheduled coverage of the visit, with Walters as chief reporter, begins at noon today with live coverage of Sadat's arrival in Israel. Walters opened last night's ABC Evening News with a satellite report from Tel Aviv.

The network has set a special one-hour report on the day's development for 7 p.m., but Washington ABC affiliate WJLA (Channel 7) decided yesterday not to carry this report. Instead the station will air its own local newscast and the regularly scheduled series "Wild, Wild World of Animals."

ABC's Sunday coverage will begin at 8 a.m. and include live one-hour speeches (to start at about 9:30) first by Sadat, who will speak in Arabic, and then by Begin, who will speak in Hebrew. The Israeli information Office is supplying all three networks with simultaneous translations of the speeches. Soverage should continue until nearly noon.

CBS coverage will begin at 12:30 today with Sadat's arrival and at 9 a.m. Sunday. The network also plans an update on the situation during halftime of the Dallas Cowboys-Pittsburgh Steelerts football game late Sunday afternoon.

A 10 or 12-minute report on the Sadat visit also will be featured on the regular weekly magazine show. "60 Minutes." If the program is not delayed by football, the Sadat segment should air at about 7:45 p.m.

At 11:30 p.m. Sunday, Cronkite will anchor a half-hour wrap-up report, "Sadat in Israel."

NBC is handling pool coverage for all three networks and will begin its own reports at 11:30 this morning with the Sadat departure from Cairo and arrival in Jerusalem.

Chancellor, the NBC anchor, will sign on Sunday at 8:30 a.m. for live coverage of the speeches. Later, NBC Nightly News will expand to a full hour, starting at 6 p.m., for reports on "World reaction" to the event, and an interview with former Secretary of State Henry Kissinger conducted by David Brinkley.

NBC also plans to expand its Monday "Today" show to three hours, from 7 to 10 a.m., for related Mideast reports and taped highlights of a Sadat news conference scheduled for earlier Monday morning.

So many media people and other interested parties are pouring into Israel that space is reportedly becoming scare. In fact, the Plaza Hotel in Tel Aviv required a deposit to hold a room for ABC's Walters, even though "They know her there," according to an ABC spokesman.

CBS and NBC spokesmen knew of no similar difficulties for Cronkite or Chancellor.