Prime Minister Robert Mugabe opened a unique conference of more than 60 nations and international organizations today with a call for almost $2 billion in aid over the next three years to rehabilitate war-ravaged Zimbabwe. A U.S. spokesman said the Reagan administration was prepared to pledge $225 million, if Congress approves.

In pledging $75 million for each of the next three years, the Reagan administration would be accepting the decision last fall by the Carter administration to triple U.S. aid to Zimbabwe, although President Reagan has sharply reduced aid to many other Third World areas and has indicated a greater willingness to support white-ruled South Africa.

The conference, an outgrowth of the British-sponsored Rhodesian peace settlement that brought black-majority rule to this southern African nation last year, is an attempt by the new Zimbabwe government to obtain a huge one-shot injection of aid for the economy similar to the U.S. Marshall Plan assistance to Western Europe after World War II.

Mugabe emphasized the importance that a stable democratic Zimbabwe could have in influencing change in white-ruled South Africa.

The week-long Zimbabwe Conference on Reconstruction and Development is also regarded as one of the first tests of the Reagan administration's attitude toward Third World countries.

Peter McPherson, new director of the U.S. Agency for International Development and head of the U.S. delegation here, told reporters he would announce Tuesday that Washington intends to provide Zimbabwe with $75 million in aid during the next fiscal year, subject to congressional approval. A similar amount, he said, would be requested for each of the following two years.

The $75 million is almost a tripling of the current aid of slightly more than $25 million a year, he noted, despite the Reagan administration's "severe austerity" program. The three-year U.S. contribution would meet about 12 percent of Zimbabwe's entire request.

Zimbabwean officials were unwilling to comment on the figure before it is officially presented, although the U.S. plan has long been known.

Former president Jimmy Carter told Mugabe of his administration's increased aid plans when Mugabe visited Washington last August. Reagan kept that figure when he presented his budget earlier this month, although aid to some developing countries was cut sharply, trimming the total package from $2.4 billion to $1.9 billion.

Mugabe, however, has frequently described the international aid received by his country as disappointing and has singled out the United States for criticism.

Zimbabwe's customary measuring stick for aid -- repeated in Mugabe's speech today -- has been the multi-nation package of about $1.5 billion proposed by then-secretary of state Henry Kissinger in 1976 and by an Anglo-American proposal in 1977. Neither peace plan was accepted.

Mugabe said the $2 billion program Zimbabwe is now seeking is limited to land resettlement and development; rehabilitation from the seven-year guerrilla war, including housing for refugees; and technical assistance.

He said the need to acquire "more productive land for thousands of our land-hungry people" is "an urgent priority item." The issue was a key to the peaceful settlement worked out by Britain in 1979 that prevented land expropriation.

Mugabe warned, "We simply have to have the resources to enable us to discharge our obligations" to that agreement "while doing full justice toward our people."

"The rural areas in the Rhodesia of yesterday," he said, "constituted a sort of colony within a colony -- neglected, exploited, underdeveloped" since the best land was reserved for the 200,000 whites.

Other speakers praised Mugabe for his handling of the birth pangs of the nation and called for generous aid.

Lord Soames, the British-installed governor during a four-month transition period leading to independence, said foreign aid would help "increase private sector investment in Zimbabwe, which is the main imperative if this country is to reach its full potential."

Claude Cheysson, representing the European Common Market, said that organization had pledged about $190 million over the next three years.

The Reagan administration's indications of shifting its Africa policy in favor of white-ruled South Africa came in for veiled criticism.

Shridath Ramphal, secretary general of the Commonwealth, an organization of Britain and its former colonies, called for concerted action to bring about black rule in South Africa and added: "Current ambivalences must surely be temporary aberrations, for to choose South Africa is to reject Africa."

Aside from the major Western aid donors, the conference is being attended by small delegations from the Soviet Union, China, Bulgaria and Romania. Yugoslavia, a key backer of Mugabe during the war, is represented by a large delegation.

Also represented are a variety of U.N. and multinational organizations.

One Zimbabwe Cabinet minister, Joshua Nkomo, did not attend the meeting. A major clash between Zimbabwe's Army and guerrillas loyal to Nkomo last month was a setback to the country's hopes to attract foreign investment.