The United States' agreement ''in principle'' to help evacuate Palestinian guerrillas from Beirut hints at the intense behind-the-scenes diplomatic efforts to bring about a peaceful resolution of the Lebanon crisis.
Diplomatic activity involving the US, France, and Saudi Arabia has reached a peak in recent days. The three nations do not agree on every detail of proposals for a solution to the Lebanon struggle. But they are working for the most part in parallel, and sometimes in close coordination.
The flurry of diplomacy has helped produce the outlines of an agreement - as much as 90 percent complete - on the withdrawal from Beirut of Palestine Liberation Organization (PLO) guerrillas. (Battling forces in Beirut are still at odds, however, as John Yemma reports on Page 8.)
But there are a number of important pieces missing. The PLO itself made it plain July 6 that it was initially rejecting the proposed agreement, which reportedly involves the use of American forces to supervise the withdrawal of PLO guerrillas from besieged west Beirut. And two other important aspects of the plan continue to defy resolution. They are:
* Where will the PLO leaders and fighters go? The current belief here is that the guerrillas would be divided into contingents destined for several Arab countries, including Syria, Iraq, Jordan, Algeria, and Egypt. One diplomat suggested that the largest number might go by sea to Syria and then split for further resettlement. But none of the Arab nations seem eager to accept the dangers involved in hosting the PLO. An even bigger problem, according to one diplomat, is that if PLO forces are scattered, the organization's leaders would feel too humiliated to accept such a plan.
* What can the PLO salvage in the way of a political position? The French and the Saudis, as well as some, if not all, US officials, are convinced that the PLO must be offered the face-saving prospect of gaining ''political rights'' once the Lebanon crisis is resolved. The French and Saudis, in particular, are convinced that Israel must agree to a gesture in this direction if Yasser Arafat , the PLO chairman, is to convince the most radical and left-leaning of his colleagues to agree to withdraw from Lebanon. In the French view, Mr. Arafat must have ''something left in his hands to show'' politically even as he loses clout militarily. So far, Israel does not seem inclined to agree.
One step in this direction would be a token PLO presence in Lebanon in the form of a PLO political office, of a PLO representative. But the Americans and French - as well as the Israelis - are agreed that the top PLO leaders must leave Lebanon along with their fighting forces. Obviously, not all Palestinians would leave Lebanon, and among those who stayed would be thousands of PLO sympathizers. But this is clearly not enough to satisfy PLO hard-liners.
In Washington, members of the Lebanese Information and Research Center, which has close ties to Maronite Christian groups in Lebanon and monitors the situation hour by hour, expressed doubts that the PLO leaders could act in a unified manner or implement any agreement on a withdrawal that Yasser Arafat signed. They, and the Israelis, seem to be convinced that Arafat and his colleagues are simply stalling, trying to buy time.
In Los Angeles, White House deputy press secretary Larry Speakes said that while ''in principle,'' President Reagan had agreed to contribute US military units if requested to do so by the Lebanese government, there has still been no agreement on such US participation. Mr. Speakes also said that there had still been no formal request for the assistance of American troops and that it would be provided only if all parties agreed to it.
The PLO, for its part, might find it difficult to agree to an evacuation on US ships or under the protection and supervision of US Marines, given the fact that they place the blame for the Israeli invasion of Lebanon on the Americans. But this is where the French could play a role. France has taken a position on the Palestinian question that is much more to the liking of the PLO than that taken by the Americans.
Informed sources said, meanwhile, that planning for a possible evacuation role for the US in Lebanon began about a week ago. In the absence of the new secretary of state-designate, George Shultz, Secretary of State Alexander M. Haig Jr. has continued to play a key role, according to these sources. He was said to have held four telephone conversations with Saudi Arabia within a single day from his long Fourth of July golf and tennis weekend at the West Virginia resort of Greenbriar.
When the Israelis began to cut off water and food going to West Beirut a protest action went from Lebanon's prime minister, Shafik Wazzan to King Fahd of Saudi Arabia. Fahd got in touch with a special Saudi envoy in Washington, Prince Bandar bin-Sultan. Bandar reached the State Department, which called the White House officials in California. They reached Tel Aviv, and managed to get the Israelis to allow the flow of water and food to resume.