Three days before a crucial presidential election, the presence of three foreign military forces as well as the cleavage between Lebanon's Christians and Moslems have raised doubts whether all the contentious forces here will allow the National Assembly's convocation Thursday, the necessary first step for the voting to begin.

Lebanon's presidency, now held by a man eager to leave--Elias Sarkis--is one of the few national institutions that has survived seven years of war. The office has remained a focus of what remains of Lebanese nationalism.

Sarkis, a Christian, was elected on a day in April 1976 when leftist Moslem forces shelled the parliament building and ambushed with machine guns the cars of deputies on their way to vote in an unsuccessful attempt to prevent a quorum and block the election.

Today, Sarkis has made it clear that he does not want his term extended for two years, a possibility if this year's election attempt ends in a total impasse.

"Sarkis is a tired, worn-out and depressed man," said a Western diplomat who has seen him recently. "There is no power with the position of president in Lebanon."

For a country that long has been torn by domestic conflict and foreign intervention, Lebanon without a president would have a substantially weakened chance to survive as an independent country with such stronger neighbors as Israel and Syria believing they have a vital stake in what happens here.

Since independence from France in 1943, by agreement of Lebanon's Christian and Moslem oligarchs, Lebanon's president has been elected by parliament after being selected from the largest of the Christian communities, the Maronites. Orthodox Sunni Moslems received the prime minister slot, and the speaker of parliament position went to a leader of the Shiites, the second largest Moslem community.

It was the challenging of this system, in part, that led to the Christian-Moslem confrontation in Lebanon's 1975-1976 civil war. The presence of the Palestine Liberation Organization in Lebanon also had exacerbated Christian-Moslem tensions, and the PLO guerrillas fought on the Lebanese Moslems' side during that conflict.

Syria then intervened with its Army. Now the Israelis have invaded Lebanon demanding that both the PLO and the Syrians leave because they are seen as a threat to Israel's security.

Most sides of Lebanon's many-sided war are now optimistic about a PLO withdrawal from Moslem West Beirut after 10 weeks of heavy Israeli shelling. But Lebanese Moslem leaders are angered about the suffering and devastation wrought by the Israelis and see the leading presidential contender, Maronite Christian Bashir Gemayel, as an ally of Israel.

Gemayel, the 34-year-old commander of the Christians' Lebanese Forces militia, may have had his presidential ambitions severely curtailed by the massive Israeli bombing of West Beirut last week. In addition, many Lebanese Moslems have a strong antipathy for Gemayel, who is seen as a hard-nosed military leader rather than a politician able to compromise.

Although he enjoys widespread support among Christians, "Moslems of West Beirut are far more afraid of Bashir than they are of the Israelis," said a well-informed Western diplomat. Lebanese Moslems see Gemayel as "a military leader who for seven years has been leading the charge against the Moslems," the diplomat continued. "Tens of thousands of people have died on both sides," he added.

But within the Byzantine maze of Lebanese politics, Gemayel might pick up some Shiite Moslem support on the basis of the belief that "the enemy of my enemy is my friend," said a longtime Western observer of Lebanese politics. The Shiites, who form a large part of Lebanon's poorer peasant class, were forced to move from their southern Lebanese farms when Palestinian refugees moved into this country and when Israel began to attack PLO bases there in the late 1960s.

Now, like a large number of Christian leaders including Gemayel, many Shiites advocate the expulsion of all the 350,000 or so Palestinian refugees in Lebanon, many of whom lived in recently destroyed refugee camps in traditionally Shiite areas.

The Shiite National Assembly speaker, Kamel Assad, has called the election for Thursday. It remains unclear if the required quorum of 62 or 66 deputies -- depending on whose interpretation of the constitution one listens to -- will come. The National Assembly has 92 members.

One of the Sunni Moslems' influential leaders, former prime minister Saeb Salam, has opposed holding the meeting -- required by the constitution -- before the PLO withdraws from West Beirut.

Known to be opposed to Gemayel's presidential bid, Salam "is a key to this election, and whatever way he goes may well determine its outcome," a Western diplomat said. The same diplomat cautioned, however, that in Lebanese politics a public posture is not always the last position.

A well-placed source in Gemayel's rightist Christian Phalangist Party, headed by his father, Pierre, contends that the parliamentary roll call will be determined "in the final 24 hours" before the Thursday meeting.

Just how many outsiders figure in this election was illustrated today when Assad returned home at 5:30 a.m. from a meeting in the neighboring Syrian capital of Damascus after meeting Syrian officials. The U.S. government is known also to look favorably on the development of a strong Lebanese presidency under Bashir Gemayel.

One obstacle to the election was removed today when both the Phalangists' Voice of Lebanon radio station and the Israeli Army radio announced that Israeli forces would withdraw from their positions around the parliament building and turn it over to the Lebanese Army. Moslem deputies would not have been willing to attend the Thursday meeting if the Israelis had remained at the parliament building.