U.S. military leaders contend that the latest confrontation between Marine and Israeli forces in Beirut is part of a larger and deliberate effort by Israeli Defense Minister Ariel Sharon to discredit the multinational force in Lebanon.

These officers, who cannot be quoted by name, said they were delighted that Defense Secretary Caspar W. Weinberger finally went public before the House Armed Services Committee on Wednesday with the latest of three confrontations between the Marines and, according to the Pentagon, one particularly aggressive Israeli tank unit.

President Reagan further heartened U.S. officers yesterday when he became more specific than Weinberger, telling reporters, "I must say that the same unit and the same commander had tried three times at this same point. And, in my view, the Marine officer did the only thing that he could do."

The president was referring to Capt. Charles B. Johnson who loaded his .45 pistol and jumped on the Israeli commander's tank in a successful effort to turn back the unit of three British-built Centurions.

The Israeli government has introduced the tank commander to reporters as "Lt. Col. Rafi," but Marine leaders have identified him to Congress as a "Col. Lansberg."

In Tel Aviv yesterday, the Israeli commander said it is "untrue" that his outfit had been involved in three confrontations.

The two earlier confrontations were reported by the Marine command in Beirut to the Pentagon but kept secret, according to defense sources. There have been several other incidents between Marine and Israeli forces which also have been kept quiet, these sources said, apparently for fear of complicating negotiations to get Israeli, Palestinian and Syrian forces out of Lebanon.

Once those foreign troops are out, the administration has said, the Marines can withdraw and leave the security of the nation to the Lebanese military. But the troop withdrawal negotiations have gone so slowly that Reagan yesterday said of the Marines' stay in Lebanon, "I can't set any time limit on it." The administration earlier had predicted that the Marines would be out by the end of 1982.

As many U.S. military leaders size up the situation, Sharon wants to demonstrate to the new Lebanese leadership, the Arab world and the Israeli public that the security of Lebanon cannot be entrusted to the multinational force, made up of Marines and British, French and Italian troops.

If the multinational force is portrayed as ineffective, this argument goes, Sharon will be in a stronger position to insist that Israeli forces remain in Lebanon, at least to man checkpoints in the southern part of the country.

As one part of this strategy, U.S. military officials contend, Sharon is ordering his forces to challenge Marine positions.

Another part, as military officers see it, is the recent claims by Israeli officials that Palestine Liberation Organization forces are using Marine lines as a shield.

If the Marines in Lebanon fail to react strongly to Israeli challenges, these U.S. professionals contend, Sharon will succeed in downgrading the Marines in Arab eyes. Marine Capt. Johnson is now being hailed over Arabic radio as the hero who "single-handedly" repelled three Israeli tanks.

Many U.S. military officers say they feel they could do a lot more to advance Reagan's peace plan if they were allowed to patrol more aggressively in Lebanon. "People have got to understand if you want us to be effective, we have to take risks and some people are liable to get killed," one said.

The administration indicated yesterday that the Marines will be kept on short leash. State Department spokesman Alan Romberg said of the tank incident, "We think it's behind us."

U.S. military leaders who have been watching the Israeli challenges for months expressed skepticism that the latest tank incident will be the last.

They noted that there also have been recent Israeli challenges against the U.N. peacekeeping force in Lebanon, interpreting these as part of Sharon's broad-gauged campaign to make Israeli troops the only foreign peacekeeping force in volatile Lebanon.