President Carter has centered his intense but thus far unsuccessful efforts to build a Middle East peace agreement this week on the concept of an interim three-way sharing of power by Arabs and Israelis on the West Bank territory of the Jordan River, informed diplomatic sources reported yesterday.

A screen of secrecy continues to surround the 12-day-old summit meeting at Camp David involving Carter, Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin. It was confirmed, however, that the idea of Israel, Jordan and local Palestinians sharing power on the West Bank for a limited period that would lead to a final decision on sovereignty has been under intense discussion here.

But Carter's efforts for agreement have been stymied by the continuing fundamental differences between Sadat and Begin over the reestablishing of Arab sovereignty over not only the West Bank, but also Egypt's Sinai peninsula and the formerly Egyptian-controlled territory of the Gaza strip, the sources said.

These sources declined to give specifics of this continuing conflict in negotiations. While there were clear indications that Egyptian and Israeli positions have significantly hardened over this point in the past three days, the sources saw this as a natural part of the closing phase of such important negotiations rather than as a sign that agreement was impossible.

Diplomatic analysts assumed that Sadat is continuing to insist on a statement of broad principles that he could interpret as including an Israeli commitment now to return all of the territories occupied in the 1967 war to Arab sovereignty eventually.

Begin, who has previously insisted on Israel's right to retain at least part of the West Bank, is believed to be resisting making such a commitment. The conflict is reported to be obstructing efforts to make the three-way power sharing a centerpiece for a final agreement for the conference.

Also unclear is what reaction the idea will draw from Jordan's King Hussein, who ruled the West Bank before the 1967 war and whose participation in continuing negotiations is seen as essential by Carter and Sadat. Essentially, Sadat needs Hussein's approval for the outcome at Camp David to be credible in the Arab world.

Sadat telephoned Hussein in London on Monday. The king, who reportedly was due to leave the British capital yesterday, has stressed in both public and private comments since that he will come into peace talks only if there is a clearly defined formula to resolve sovereignty on the West Bank and in Gaza.

Again refusing to discuss any details of the talks or to suggest when they might end, White House press secretary Jody Powell yesterday denied that the summit conference almost collapsed Thursday night. Egyptian and other press reports overnight had suggested that Sadat had threatened to break off negotiations described as totally deadlocked.

Powell said the summit would last at least until Sunday, and possibly longer. He repeated his view that "more flexibility" remains necessary if the talks are to produce the U.S. objective of a "framework" for an Arab-Israeli peace.

Briefing reporters at the American Legion hall six miles east of Camp David, Powell confirmed that the sticking points in the negotiations involve not only the West Bank but also the Sinai. Israeli civilian settlements and security buffer zones have been points of dispute in the past.

In Tel Aviv, the director general of Prime Minister Begin's office, Eliahu Ben Elissar, told a meeting of American Jews that Israel was prepared to recognize Egyptian sovereignty over all of Sinai if the Arabs withdraw their insistence that Israel pull back from every inch of the other territories occupied in 1967, news agencies reported.

Powell announced that Carter held his regular weekly foreign policy breakfast with Vice President Mondale and senior presidential advisers before meeting Sadat for 30 minutes. Begin started observing the Jewish sabbath at sundown and will not participate in talks today.

The continuing absence of any negotiating sessions between Sadat and Begin, who last met in formal talks on Sept. 7, elicited questions about possible tensions between the two. Powell said only that he had no reports of personal hostility.

Initial reports of what now appears to be the first significant leak of the conference came in the Israeli afternoon daily Yediot Aharanot, which said that under the West Bank power-sharing concept, Jordan would take over local police powers, issue passports and control education.

Leaders of the 1.1 million Palestinian Arabs who inhabit the territory would run domestic affairs, along the lines of the "autonomy" plan that Begin offered as a basis for negotiations in December.

Israel would maintain tropps on the West Bank and be in charge of security for the interim period.

An informed source who provided an independent description of the concept said that Israel appeared to be pressing for some residual security presence and the right of individual Jews to settle on the West Bank after the interim period, and still insists on leaving the question of eventual sovereignty open.

The sudden hardening of positions appeared to have been reflected in House Speaker Thomas P. (Tip) O'Neill's decision to "scrap" tentative plans for a joint session of Congress Monday to hear a report from Carter on the summit. O'Neill said that he had received inquiries about the possibility of holding such a session and then had been advised by the White House to drop the idea for now.