As the Falkland Islands crisis enters its third month, questions persist about whether the United States knew that Argentina planned to seize the islands and gave a tacit go-ahead to ensure the Argentine military junta's cooperation with U.S. campaigns against guerrillas in Central America.

They are questions that, if not resolved, are likely to add a major new dimension to the intense controversy over President Reagan's efforts to cultivate the friendship of military regimes as the cornerstone of an inter-American front against communist penetration of the Western Hemisphere.

So far, the Reagan administration has not responded in any detailed public manner to speculation about its role in the maneuvering that went on prior to Argentina's April 2 seizure of the islands from Britain.

But, from what can be learned from well-informed sources here, the idea of advance U.S. knowledge or complicity appears to be largely a myth. A case can be made that the administration, through intelligence failures and misjudgments about Argentine priorities, missed several opportunities to make its views so unmistakably clear to the junta that the bloodshed taking place in the South Atlantic might have been averted.

However, the sources unanimously agreed that the United States did not know Argentine intentions because the invasion, conceived during the early months of this year, was a well-guarded secret, known only to President Leopoldo Galtieri and the inner circle of the ruling junta, plus one civilian cabinet minister, Foreign Minister Nicanor Costa Mendez, and a few lower-ranking officers needed to plan its mechanics.

According to the sources, even the great majority of top commanders in the Argentine armed forces were kept in the dark until the time that the operation was ready. Given that emphasis on secrecy, the sources insisted, the junta had no intention of compromising its plans by revealing them to the United States or other foreign governments.

Instead, the sources added, the junta, relying on the advice of Costa Mendez, made several assumptions about how Britain, the United States and the Soviet Union would react. Essentially, it assumed that Britain would not resort to military action, that the United States would talk the British into accepting some face-saving concessions and that the Soviet Union, sensing a chance to strengthen its ties with Argentina, would veto any British attempts to obtain redress through the U.N. Security Council.

However, the sources said, these assumptions, all erroneous, were based on a combination of deduction and wishful thinking that some characterize as "totally divorced from reality."

What's more, the sources continued, the junta's unwillingness to accept that it had miscalculated and to seek to cut its losses through negotiation has remained the principal impediment to a halt in the fighting. Secretary of State Alexander M. Haig Jr.'s shuttle mediation failed because the junta, despite repeated warnings from Haig, simply refused to believe that the United States would openly support Britain.

Even now, when most military observers believe that the fighting has tipped decisively in Britain's favor and that the Argentines are about to be forced off the islands, the sources contend that the junta is paralyzed by its mistakes and pressures on it by angry factions of the armed forces excluded from the original invasion plan. As a result, the sources believe the junta is incapable of any action other than standing back and fatalistically awaiting what is likely to be a humiliating defeat on the islands that will topple it from power.

Still, the speculation about whether the junta originally acted in the belief that it could count on the Reagan administration's support has continued. To a large extent, it has been fueled by Argentine officials who have told reporters and diplomats in background briefings that the junta had made clear to the U.S. administration the high priority it placed on reclaiming the Malvinas, as Argentina calls the islands, and had predicated its strategy on the belief that Washington would intercede on its behalf against Britain.

In this country, the idea that Washington had at least some advance inkling of Argentina's intentions and reacted with "a wink and a nod" has been the subject of discussion among many liberal academicians and human-rights activists.

However, that charge is disputed by a variety of sources familiar with the course of U.S.-Argentine relations in the Reagan administration. Although these sources insist on anonymity, their accounts, obtained in separate interviews, dovetail closely. Collectively, they sketch this picture:

When President Reagan took office, one of his first major foreign policy moves was to begin reversing the activist human-rights stance associated with President Carter. That policy had made Argentina a virtual pariah because the Argentine military, during the 1970s, had moved against leftist terrorists with its infamous "dirty war" that saw thousands of people literally disappear as the result of arrests and kidnapings.

Instead, the Reagan administration put top priority on countering leftist guerrilla movements in El Salvador and elsewhere in Central America. As it searched for allies, it immediately began mending fences with Latin American military regimes in accordance with the theory, put forward by U.N. Ambassador Jeane J. Kirkpatrick and other policy makers, that "authoritarian" governments, unlike "totalitarian" communist states such as Cuba, could be weaned gradually toward democracy.

Kirkpatrick, in particular, is understood to be very upset at the rupture that the Falklands situation has caused in attempts to build a special relationship with Argentina. According to the current issue of Newsweek magazine, she and Haig had a bitter, 45-minute telephone exchange last week in which she reportedly accused the secretary of being too slanted toward Britain and insensitive to U.S. interests in Latin America.

Initially, Washington saw Argentina as ideally suited to playing a leadership role in U.S.-sponsored strategic ventures ranging from naval vigilance over the South Atantic to the support and training of anti-communist forces throughout Central America.

Galtieri was viewed as a particularly valuable ally. He was regarded as a moderate seeking to curb the excesses of the armed forces, had strong anti-communist credentials and openly advertised his eagerness to align Argentina more closely with the United States.

He also had specific ideas about consolidating his domestic power and popularity to the point that he would be able to smash the opposition of the Peronists that are the country's major political force, force Argentina to hold still for a long and painful period of surgery on its ailing economy and eventually return the nation to civilian government, with himself the favored candidate to be elected president.

What wasn't known was that he decided to redeem Argentina's 149-year claim to sovereignty over the Falklands. Shortly after becoming head of the junta, he and his cohorts began planning to make that dream a reality.

Parallel to the military planning was the political strategy based on the advice of Costa Mendez, an experienced and worldly diplomat who was regarded by the junta leaders as having a good understanding of the United States and Britain. He is understood to have advised the junta leaders that if the Falklands could be captured without death or injury to the 1,800 residents, Britain would not retaliate militarily and that the United States, seeking to keep Argentine favor, would block Britain from seeking political or economic sanctions.

Despite the secrecy, hints of what was being planned did surface. For example, on Jan. 24, the influential Buenos Aires newspaper, La Prensa, which has close ties to Costa Mendez, published a lengthy commentary predicting a high-priority effort to recover the Falklands. It said Argentina wanted "something beyond the mere recovery of a portion of its sovereignty" and added:

"As far as we know, Washington understands it so, this being the reason why it reportedly has expressed its support for 'all of the actions' leading to the recovery, without excluding military actions."

According to the sources here, such hints of U.S. acquiesence, while apparently planted by the junta, do not square with the facts. These sources insisted that, while a number of key administration officials visited Argentina in the past year, at no time was any sign given to them that Argentina would resort to military action.

The sources conceded that, during these visits, the Falklands were mentioned frequently by the Argentines, but the U.S. officials tended to view it as no more than restatements of long-held Argentine positions. In fact, some of the visiting Americans are known to have received the impression that if there were any danger of precipitous action over a territorial dispute, it would have been directed not against the Falklands but at the dispute with neighboring Chile over the Beagle Channel.

In early March, Thomas O. Enders, assistant secretary of state for inter-American affairs, visited Buenos Aires, and Argentine officials say an effort was made to impress upon him their insistence that the Falklands issue be resolved. Again, the sources here contended that the facts are different.

They say that Enders, before making his trip, was contacted by the British Foreign Office and asked to urge the Argentines to resume negotiations over the Falklands that had taken place in New York in February. Enders did raise the subject with Costa Mendez, who was described by the sources as having given a noncommittal but not negative reply.

While there, Enders also was briefed on the Argentine position on the Falklands by foreign ministry officials. But that was described as containing no hint that the situation was approaching the stage where Argentina would take military action less than a month later.

When the Argentines started moving at the end of February, the sources said, it surprised not only the United States but also the intelligence services of Britain, Chile and Brazil.

At Britain's urgent request, Washington tried to head off the invasion, first through representations at the embassy level and then by Reagan's now-famous phone call to Galtieri. On each of these occasions, the sources said, the United States made unmistakably clear that an invasion would mean the end of the developing U.S.-Argentine friendship, but the Argentines reacted each time as though they did not believe Washington would carry though on the threat, an attitude they would maintain through all the subsequent negotiations with Haig right up until April 30 when Reagan stunned them by publicly siding with Britain.