Three weeks after seizure of the U.S. embassy by Iranian militants, there is growing evidence the long-running crisis may be one of those rare international "hinge events" that change the way people think and government act, therefore altering the course of history.

The American government, beset by the draining demands of day-to-day maneuver, is just beginning to look beyond the tactical situation to the broader repercussions. It is increasingly clear, even in the first phase of studies the State Department recently ordered, that the post-crisis world will be different than the world of three weeks ago, regardless of the final outcome.

Two international events of the last dozen years, in the view of this reporter, were of a class by themselves in the powerful tremors and upheavals they generated. About five years apart, these events were the 1968 Tet offensive by communist forces in Vietnam, which changed the minds of Americans about the Indochina war and was the turning point of U.S. involvement in faraway military conflict, and the 1973 Middle East War and oil embargo, which changed the terms of trade in the world and inaugurated the age of Arab oil power.

As another Potential hinge event, the U.S. Iranian clash of 1979 may resemble Tet more than the oil embargo because its most direct effects are on minds of people rather than material commodities. But the world's petroleum supply, as well, is ultimately at risk.

Like Tet, near-simultaneous television coverage via satellite of an invaded U.S. embassy in Tehram brought shocking symbols as well as substance into the living rooms of the American people. Unlike Tet, the immediate public and political outcry has been held in check by concern for the hostages, even while fierce emotions continue to seethe just beneath the surface.

As did the coordinated Tet holiday attacks on nearly every city and town in South Vietnam in January 1968, the Iranian crisis caught the American body politic in the early stages of a hotly contested presidential election struggle involving an embattled incumbent. This is a moment of unusually great potential for change.

In recognition of the likely impact on their political fortunes nearly every candidate, including President Carter, has begun considering or drafting a speech to be made the day after the American hostages in Iran are released, according to a well-placed administration official.

Should American hostages not be released and the episode end in bloody and tragic fashion, the consequences would be so grave at home and abroad that it is difficult to contemplate them. Officials are reluctant to discuss the possibilities in this case even in sketchy outline.

Amony the many other high visibility acts of terrorism and flashpoint conflicts in recent years, the Iranian crisis stands out as different and more disturbing in several ways:

First it is the first in modern times that the seizure of a diplomatic mission and its personnel has been sanctioned, if not instigated, by the host government, in open violation of international rules and practice.

Second, the occupation of the Tehran embassy is among the longest running international hostage situations to capture worldwide attention. For three weeks now and with no end in sight, the normal business and diplomatic priorities of the United States, a great power with heavy responsibility for world order, have been subordinated to the plight of the hostages despite many other situations of great importance in the world.

Third and most confounding is the complicated tangle of vital interests and problems affected by the increasingly hostile relationship, verging on undeclared war, arising from seizure of the embassy and the Iranian demand for extradition of the deposed shah, Mohammad Reza Phlavi.

The conflict has economic, political and military dimensions. It involves oil, international finances, the future of the strategic Persian Gulf, the consciousness and direction of the Islamic world, relations of the United States with the Third World of developing nations and the overlapping interests of the United States, the Soviet Union, Europe and Japan.

To approach an understanding of the stakes involved, it is necessary to look more closely at the extraordinary factors in the current crisis.

The fear in Washington's high councils is that the first government-sanctioned embassy takeover may be like the first hijacking of an international airliner for political hostage-taking purposes, by Palestinian commandos in 1968. In other words, it will set a pattern.

A substantial number of the 141 U.S. embassies and 142 consulates and other U.S. diplomatic posts abroad arein areas of instability where host governments are not always friendly or in full control.

In the past few days Washington has appealed to many Islamic or Middle East-South Asian nations to beef up their protection of U.S. facilities and to designate quick reaction forces for embassy protection with established lines of liaison to U.S. diplomats. As the attack on the Islamabad embassy illustrates, protection is neither easy nor automatic.

This could mean that U.S. embassies in many areas may have to be more heavily fortified and U.S. guard forces increased. Wives and children of more diplomats may have to be kept at home. The number of diplomatic missions in the field may have to be reduced, forcing back to Washington more of the business with and assessments of foreign countries.

Another set or problems and decisions is posed by the length of the Tehran embassy crisis. In a world of instant international mass communications, public impatience and speeded up events, time is a strategic dimension. Three weeks of soaring tension seemed inconceivable at the start of crisis.

Three more weeks or even more seems an almost intolerable prospect today, yet no positive movement is expected before next weekend and no clear sign of a break is in sight even then.

Until now, the president has canceled his out-of-town travel and kept other business to a minimum in intense concentration on the Iranian crisis. His senior foreign policy and military aides have thought of and worked at little else. Official Washington has been in its crisis mode, with Iran dominating private talk and public news.

The administration is now approaching a decision on how to proceed if the crisis is further protracted. Can it better maintain a semblance of control over the domestic mood and international developments by shifting to more normal gear and reducing the tension level, or does it improve the prospects for the hostages by maintaining the present intensity? This is being debated on high.

With all that is at stake, there is no question of "a business as usual" posture. The lives of 49 Americans in the embassy compound plus those of acting ambassador L. Bruce Laingen and two aides at the Iranian foreign ministry in Tehran are at risk, as is the honor and international standing of the United States.

As the crisis has developed, crucial international questions have arisen:

What will be the impact on the world's tight energy supply of continuing militancy bordering on hysteria of one of the world's major petroleum powers, formerly the second largest oil exporter and owner of about one-seventh of the oil cartel's total productive capacity? Europe and Japan far more than the United States have been dependent on Iranian oil.

Can the delicate international financial system sustain the withdrawal of Iranian participation and the seeming decision to repudiate its foreign debt? The best selling novel, "The Crash of '79," contemplated a financial crash based on manipulation of Iranian assests.

Will the U.S. freeze of Iranian official assets shift the internal argument in Saudi Arabia against continued high levels of oil production? Saudi oil is exchanged for paper dollars and dollar balance that could be similarly impounded in a crisis. Treasury Secretary G. William Miller is in Saudi Arabia today to make the case against such thinking.

Will Ayatollah Ruhollah Khomeini's passionate appeals to the Islamic masses shake the internal foundations and external alliances of conservative Islamic governments in the Persian Gulf and elsewhere in the Middle East? Will a Khomeini inspired Islam-versus-American polarization make the U.S. presence less acceptable in the strategic region?

Can the Soviet Union, which borders on Iran, is a purchaser of large amounts of Iranian natural gas and has an increasing thirst for energy resources, restrain its instinct for a geopolitical big score as Iran's revolutionary upheaval continues? And can the Russians tolerate continued hints of U.S. military intervention in Iran if the hostages are harmed, or actual military operations should it come to that?

Probably the greatest question concerns the attitudes of the American public as the crisis unfolds. The Symbolism of a great nation as impotent hostage to threatening foreigners has the potential for a powerful domestic reaction, the more so because the event is so mystifying and the motivations and objectives on the other side so little understood.

Another symbol, of the anti-American Khomeini as the embodiment of the Third World, could bring an emotional and irrational swing in domestic attitudes toward the gravely troubled, nationalistic and increasingly vocal nations of the developing world. Long before the imam returned from exile to Iran, American-made political and economic arrangements were under fire in this world.

What flows from the ordeal of the American diplomatic hostages in Tehran is likely to transcent the conflicts which brought it about or circumstances which bring it to an end.

The danger is plain that this "hinge event" can make the world riskier for everyone, that both the relations of nations and the management of resources will be even more difficult when the crisis is over.