Beirut — Three unexpected developments over the weekend have led to speculation that the United States may once again be considering diplomatic intervention to help boost peace efforts in Lebanon.
* Assistant Secretary of State Richard Murphy, who flew to Beirut after the US Embassy annex bombing, made an unexpected and unannounced detour to Syria Saturday and Sunday. It was the first visit by a high-ranking US official since the Reagan administration dropped its Lebanon peace initative.
* Israeli Foreign Minister Yitzak Shamir told the New York Times that Israel no longer demanded the simultaneous withdrawal of Syrian troops from eastern and northern Lebanon as a precondition for pulling out Israel's forces occupying the southern third of Lebanon. And he said Israel might ask for US mediation with Syria on terms for Israeli withdrawal.
* In a speech marking the second anniversary of his presidency, Lebanon's Amin Gemayel said rival factions must unite to ensure the withdrawal of foreign forces. He also announced that Lebanon would soon begin consolidating security forces to ensure an Israeli withdrawal.
Almost overnight, it appears that after two long and troubled years, there may be fresh hope about ending the occupation of Lebanon. American sources said Mr. Murphy's trip had been planned from the outset as a ''reasonable extension'' of his Lebanon visit. Yet it was so sudden that a US helicopter had to be hastily summoned to ferry him to Cyprus and then to Syria, since there are no air connections between Beirut and Damascus.
He had only been in Lebanon for 24 hours, mainly to investigate last Thursday's bombing. And his last act was a press conference where he made no mention of the Syrian junket.
Mr. Murphy, a former ambassador to Syria, is highly respected in Damascus, mainly because he said Syria was playing a ''helpful'' role in Lebanon in testimony last July before a House subcommittee.
It marked a major change in US attitude, if not policy, toward the regime of President Hafez Assad, who had repeatedly been blamed by the White House and State Department for blocking peace efforts in Lebanon. Until Mr. Murphy's statement, relations between the US and Syria had been, at least, icy.
The Reagan administration had also listed Syria as one of the nations responsible for the wave of ''state-sponsered'' terrorism against US diplomatic and military installations. There was immediate widespread speculation that Mr. Murphy would bring up the foreign troops withdrawal issue in Damascus, as well as attack on the embassy annex and the kidnapping of three Americans in west Beirut over the past seven months.
The Israeli statement marked a major turn of events and is a virtual reversal of their position during the US-orchestrated negotiations last year. The move would appear to open the way for renewed mediation, and possible Syrian acceptance.
Although Mr. Assad had earlier pledged withdrawal of his troops, he then rejected the so-called ''May 17'' agreement between Lebanon and Israel last year. He argued that Syrian forces had been invited to Lebanon as peacekeepers in 1976, whereas the Israelis invaded. Therefore the Damascus regime refused to have its troops dealt with in the same manner as the Israelis.
It was the Syrian position that subsequently led Lebanon to abrogate the deal with Israel - and diplomats to fear Israeli forces might remain in Lebanon indefinitely. Mr. Shamir's statement clearly reflects the policy of the new coalition government to bring the troops home as soon as possible. Meanwhile, international sources in the south report a reduction in Israeli troop strength in recent weeks.
President Gemayel's speech added grist to the mill, since security guarantees for Israel's northern border will continue to be a primary condition for withdrawal.
There had been serious doubt expressed by foreign military sources about the ability of Lebanon's Army and Internal Security Forces to secure the south after an Israeli pullout. The main concern had been infiltration of Palestinian guerrillas back into their former strongholds.
Mr. Gemayel appeared to be signaling that his nation would make a new effort to provide guarantees so the Israelis would depart and Lebanon could be reunited.
His speech also amounted to an appeal to the various warlords, many of whom are now represented in the five-month-old ''national-unity government,'' to end the squabbling that has delayed constitutional reforms and progress on reconcilation after almost 10 years of war.
Although the three events by no means indicate a breakthrough, diplomats in Lebanon agreed that there was, for a change, a small basis for optimism.