U.S. Marines sent here as part of the multinational peace-keeping force have settled in at the Beirut international airport, where they are the least visible, and probably the least busy, of the three contingents making up the 3,950-member body.

Their precise role in Beirut is unclear. But it seems to involve the hope that their mere presence will help the Lebanese Army regain confidence after seven years of doing virtually nothing.

In fact, each of the three participating nations appears to have defined its mission -- and decided what it is willing to do on the ground -- in a separate and somewhat different way.

The 1,650 French troops can be seen all over the capital, in the Christian eastern sector as well as the Moslem western one, riding in jeeps, trucks or armored cars or just standing at street corners in twos and threes.

Their obvious intent, as French officials readily confirm, is to restore a sense of security to the civilian population. But they are also actively aiding the Lebanese Army to reassert its authority over the capital -- to the point of searching cars for arms at checkpoints.

The 1,100 Italian Marines and paratroopers are concentrated in and around the three Palestinian camps of Sabra, Shatila and Burj al Barajinah and thus are bearing the brunt of the peace-keeping force's main reason for coming here, to protect the Palestinians from the recurrence of the massacres inflicted on them by Christian militiamen Sept. 16-18.

The Italian government, however, has refused to allow them to get involved in security checks of cars and people.

In contrast to the high profile of the French and Italians, the presence of U.S. troops is scarcely noticeable, even at the international airport.

Two Marines standing alongside a few Lebanese Army soldiers at the last checkpoint before the airport entrance and several others posted at the gates of a nearby compound serving as headquarters are among the few signs to the Lebanese public that there are 1,200 American Marines inside.

There are also helicopters shuttling back and forth between the airport and several ships standing within sight offshore and a landing area guarded by Marines at a nearby beachfront.

Col. James Mead, the 47-year-old commander of the 32nd Marine amphibious unit, said it was "with difficulty" that he could answer the question, "how do you describe your mission here."

"It's rather a unique one of presence not taught at our military schools," he began with a smile.

The idea, he said, is to help establish "the proper environment" in which the Lebanese Army can reassert its badly bruised authority at the airport and in the capital.

"As they exercise their authority, we merely remain there as a presence," he said.

Above all, the Marines are not involved in providing "security" at the airport, a task Mead said was solely that of the Lebanese Army.

"We are not in effect defending anything," he said. "We are just standing there actively watching to see that the stability in the area is maintained."

If the Marines see something unusual, such as unidentified armed elements, the information is passed on to the Lebanese Army and up the peace-keeping force's command, Mead said. "But it is up to the Lebanese armed forces to take action in that regard," he added.

So far, there has been just one such incident, a group of Christian militiamen sighted outside the Marines' area near the Lebanese University east of the airport.

The Marines were called into the university, which was the scene of fierce fighting last June, to clear mines from the campus grounds and defuse other unexploded ammunition.

Col. Mead said his troops had already discovered everything from 155 mm shells to the deadly cluster bomblets, known as "birdies" -- one of which exploded the second day the Marines were here, killing one of them and wounding three others.

The colonel, who stands six feet five inches, was playing with one of the golf ball-sized "birdies" -- presumably disarmed -- throughout the interview and demonstrated in detail how it worked. The bomblets found here, most of them American made, were scattered from bombs or 155 mm shells provided by the U.S. government to Israel.

Outcry over the use of them here in Lebanon provoked President Reagan to suspend their sale to the Israeli Army, although press reports here say that only supplies of bomb casings were actually held up.

In addition to settling in and observing, the Marines' main activity is clearing mines from across the airport grounds, on the coastal road alongside it and at the university.

Col. Mead said the Marines had so far uncovered 700 pieces of ammunition, 200 of them in caches. The 50 types of munitions recovered, ranging from 155 mm shells to blasting caps, were from nine nations.

Mead said that so far the Marines, in their nine days here, had encountered "no trouble whatsoever" and described the working relationship with the Lebanese Army as "wonderful."

"There is a feeling of rebirth and hope," he said, noting the incredible Phoenician trading instinct of the Lebanese. "We saw a mobile 'Joe Marine shop' selling everything when we arrived at the port," he remarked with a laugh.

The largest apparent problem for the Marine force is finding quarters. They are presently camped in shell-blasted and burned-out buildings in and around the civil aviaton center and fire-fighting school adjacent to the main terminal.

There are three companies living in tents along the southeastern side of one of the airport's crisscrossing runways and other units along the southern edge, where they overlook an Israeli unit about 300 yards away.

Mead said he had no contact with the Israeli soldiers. At first the Israeli unit had pointed two tanks towards the Marines, and negotiations to remove the tanks were carried out through diplomatic channels rather than directly. Today, the tanks were finally gone, following initial Israeli refusal to remove them.

Mead said he is looking for more permanent quarters in tents with wooden floors, now that the rains are beginning here and there is no sign of an early departure.