Described as fit and remarkably "upbeat" by U.S. Air Force doctors, the 39 freed American hostages here "held another one of our famous meetings," as one ex-hostage put it, and most decided to return to the United States Tuesday afternoon.

They are scheduled to arrive at Andrews Air Force Base at 3 p.m. and will be greeted by President Reagan.

As the final phase of their journey from captivity in Beirut to freedom at home approached, events linked to the hijacking 18 days ago of the Trans World Airlines jetliner by Shiite extremist gunmen continued to unfold in world capitals.

In Jerusalem, Defense Minister Yitzhak Rabin announced after a Cabinet meeting that Israel would free about 300 of the 735 mostly Lebanese Shiite Moslem prisoners held at the prison camp at Atlit, Israel, within 48 hours.

Release of all those prisoners had been the hijackers' main demand. But both the United States and Israel have stressed that they made no deals to free the hostages, and Israel had said before the hijacking that it intended to release those prisoners gradually as security conditions in southern Lebanon permitted. Nevertheless, the new release was viewed in Israel as underscoring the widely reported "understanding" with Washington that no more detainees would be released until the Americans were safe.

In Damascus, there were signs that the Syrian government of President Hafez Assad, who played a key role in finding a solution to the hostage crisis, has been displeased by the remarks and actions of the Reagan administration in the aftermath of the crisis, especially what the Syrians view as a lack of gratitude in Washington.

Assad abruptly canceled a long-planned trip to Czechoslovakia that had been set for Tuesday in part, his aides said, because of the situation in the Middle East. Western diplomats in Damascus also questioned U.S. tactics, as Washington was hoping to enlist Syrian help in freeing another seven missing Americans who have been kidnaped in Beirut during the past year.

In Washington, the State Department announced that the United States is taking both legal action and diplomatic steps to "isolate Beirut International Airport" and to encourage others to do so. The idea, the department explained, was to pressure Lebanon to combat terrorists who use the airport.

Lebanon's Middle East Airlines will have to end its twice-a-week flights to New York, something that an airline spokesman said would jeopardize all 7,000 jobs on the struggling air carrier. Lebanon's ambassador to Washington said he will deliver a formal protest to the State Department on Tuesday, claiming that steps against the airport will punish the Lebanese government and people, who condemned the hijacking, and not hurt the terrorists.

In Madrid, two terrorist attacks against a building housing offices of TWA and British Airways and against the Jordanian airline killed one person and injured 27 others. A caller to a news agency in Beirut claimed that the TWA bombing was the work of the "Organization of the Oppressed" and was in response to Reagan's threat to strike against terrorists.

Vice President Bush, after greeting the hostages here last night, flew to Paris to meet with French Prime Minister Laurent Fabius. In one sense, the two countries have been drawn closer together because four French citizens are still missing in Lebanon along with seven American kidnap victims.

But while the Reagan administration has called for concerted western action against terrorism, French officials said privately that they prefer quiet, bilateral diplomacy to improve airport security.

At the big U.S. medical facility here, the 36 passengers and three-man crew of the hijacked TWA jet who had been held hostage for more than two weeks by Lebanese Shiite militiamen were pronounced in excellent physical and mental condition after preliminary tests by a U.S. Air Force medical team.

Many former captives, who were joined by dozens of relatives here today, said they were eager to depart for the United States on Tuesday, a day earlier than expected.

"They are in excellent condition; it's a very upbeat group," said Col. Charles K. Maffet, commander of the U.S. military hospital here and head of the eight-man team of physicians examining the hostages.

The strong camaraderie of the group appeared to have aided many of the hostages in coping with the anguished uncertainty of their fate. "They clearly gained a lot of support from each other during this crisis," Maffet said.

Despite their harrowing ordeal aboard the hijacked TWA flight, the hostages appeared to have been fed and treated well by their keepers, according to Maffet. He said there were no signs of physical abuse, and the only injury appeared to be a bee sting suffered by a member of the TWA crew.

Final results from the medical examinations, which have included electrocardiograms and blood counts, were expected within 24 hours. Maffet stressed that as private citizens the hostages were free to come and go as they pleased, and noted that several of them intended to stay with relatives in hotels this evening while awaiting flights home.

Hospital sources said that the youngest ex-hostage, James Hoskins Jr., 22, left the hospital in the company of a woman today, carrying his suitcase and headed for an unknown destination, United Press International reported. There was no official word on whether he had quit the voluntary medical checkup.

FBI investigators and members of the State Department's counterterrorism squad began debriefing some of the 39 Americans today to learn more about the hijackers.

One hostage, San Franciso nightclub owner Jack McCarty, said the FBI questioned him for about two hours today seeking information that might shed light on the fate of the seven remaining Americans held captive in Lebanon.

"We want to learn how it happened and who really bears ultimate responsibility," a State Department official said.

The FBI has become involved in the case because last year Congress passed legislation making the hijacking of U.S. aircraft anywhere in the world a federal crime.

U.S. officials said they were not surprised by some of the relatively favorable comments made by the hostages about their keepers during their captivity and in the initial hours after their release.

"It's interesting to hear some of their opinions now," said a source close to the counterterrorism investigators. "But we want to see if they express the same views later on in a less coercive atmosphere." Later in the day, there seemed to be some changes in tone, with some former hostages suggesting that they may have gone too far in discussing the "hospitality" of their captors.

Thomas V. Cullins said: "In retrospect I always thought that I made a mistake in that first press conference by using the word hospitality. I was embarrassed by it personally after that, frankly."

Jimmy Dell Palmer, who was released for medical reasons in Beirut said, "In the most part I agreed with Allyn [Conwell, the unofficial spokesman for the group]. But toward the last, I was beginning to get the feeling that he was slipping a little bit too much toward their side."

Some of the released Americans were reunited with their families at the hospital, where they enjoyed a hearty ham and egg breakfast together. Others went to their rooms to shower and to talk to loved ones back home on telephones set up only for outgoing calls.

Maffet said that because of their short captivity, the TWA hijacking victims should suffer few transition problems. He said their readjustment should prove much less difficult than it did for the Tehran hostages, who were held for 444 days.