Egypt expressed strong reluctance today to accept a U.S. invitation for three-way talks in Washington on the dispute about Sinai observer forces, saying the problem is only between Israel and the United States.

Officials indicated, however, that Egypt is unlikely to block the talks.

The State Department, meanwhile, said today that it is "unlikely" that the meeting will be held this week as oiginally planned because of previously scheduled meetings of Israeli and Egyptian officials in Israel. But it indicated that the United States is confident the meeting will take place.

A State Department official and in Washington that while a formal invitation has not been extended to Egypt, U.S. officials had discussed the meeting with Sadat's aides and felt Egypt will participate.

Meanwhile, President Carter and U.N. Secretary General Kurt Waldheim met in Washington today to discuss the plan to use U.N. observers to police the phased Israeli withdrawal from the Sinai. The White House issued no details of the 50-minute meeting.

The Egyptian stand flowed from President Anwar Sadat's evident desire to avoid new friction with the Israelis and his contention that, having accepted a U.S. suggestion for Sinai observers, Egypt has removed itself from the disagreement.

Egypt's Deputy Foreign Minister Boutros Ghali said that against this background Cairo could see "no utility" in a meeting between Israel, the United States and Egypt as suggested during the weekends by the State Department.

In addition, Egyptian officials pointed to the fact that Washington orignially scheduled the Sinai conference at the end of this week, when Premier Mustafa Khalil normally would be preparing for the next round of Palestinian autonomy talks set to open Sunday in Israel.

Khalil, who also is foreign minister, would be the logical Egyptian envoy to Washington, but he heads the Egyptian autonomy team. Israel has announced its intention to dispatch Foreign Minister Moshe Dayan and the U.S. invitation was made in the name of Secretay of State Cyrus Vance.

Ghali said Egypt is seeking "clarification" from Washington on what the suggested talks would embrace. This was interpreted as a polite way to stall and convey the signal that Egypt would rather stay out of the argument.

Other Egyptian officials suggested, however, that if pressed by Washington, Sadat would be likely to relent and send a lower ranking envoy, possibly Ghali. The Egyptian defense minister, Gen. Kamal Hassan Ali, is in Israel for talks with his Israeli counterpart, Ezer Weizman.

The Egyptian reluctance to step into Sinai dispute was seen as another indication that Sadat does not view it as a major problem that would slow Israeli withdrawal from the Sinai as laid down in the peace treaty signed March 26 in Washington.

When the disagreement first arose last week, Sadat dismissed it as a "side issue" and predicted it would be resolved within a few days. There are some indications that he underestimate its importance to the Israelis and therefore to the United States.

Diplomats here pointed out that the good feelings between Sadat and Israeli Prime Minister Menachem Begin may have led Sadat to believe the solution would be easier than it is proving to be. In the same spirit, Weizman suggested that perhaps Israel and Egypt could arrange their own patrols of the pullback lines, skipping the international force foreseen in the treaty.

But Egypt has insisted on keeping the observer forces under the aegis of the United Nations. Egyptian officials proclaimed they were satisfied with the U.S.-Soviet compromise of last week, under which the current U.N. Emergency Force would be replaced by the U.N. Truce Supervision Organization.

Israel objected, however, pointing out that the peace treaty spoke of "forces" whereas the UNTSO troops are unarmed. In addition, Israel contended the Sinai observer forces should be under Security Council authority and not that of Waldheim as are the UNTSO observers.

A top Waldheim aide, Undersecretay General Brian Urquhart, said here today that, in principle, unarmed observers such as the UNTSO troops should suffice to monitor the withdrawal. But, speaking with reporters, he insisted he was in Cairo only in connection with the pullout of UNEF troops whose mandate has expired and not as part of the debate about who should replace them.

The UNEF troops, who are armed, have been ordered to leave after five years of monitoring Sinai truce lines. The Soviet Union threatened to veto renewal of their mandate in the Security Council to avoid giving the appearance of endorsing the Egyptian-Israeli treaty opposed by its Arab allies.

Amid the efforts to win agreement on their replacement, two working groups of Palestinian autonomy negotiations in Alexandria wound up a two-day session in what a Egyptian spokesman described as discord.

Egyptian and Israeli envoys "sharply differ" on the subjects discussed by both working groups, he said. These are the organization of elections in the occupied West Bank and Gaza and the definition of autonomy to be exercised by self-governing bodies established in the vote.

The results of the working groups' discussions, the spokesman added, will be put before the full autonomy negotiating teams at their talks opening in Israel Sunday. CAPTION: Picture, EZER WEIZMAN . . .proposes own patrol of Sinai