Treasury Secretary James A. Baker III today outlined a three-point plan to ease the world's debt crisis, but the proposal was less ambitious than the Reagan administration originally intended to offer to the world's finance ministers.

Baker told reporters in a briefing before his first address to the joint meeting of the International Monetary Fund and World Bank here that the United States plan would:

*Push commercial banks to boost their lending by about $20 billion over the next three years to cash-strapped countries.

*Call upon the World Bank and other multinational lending organizations to boost their lending by about $9 billion during the next three years.

*Encourage the debtor nations themselves to adopt more efficient internal economic policies that are geared more to the market and less to the state.

"The three major players -- the debtors, the private banks and the multilateral development banks -- have to sit around the table and work this out if we are going to solve the problem" caused by the world's mushrooming debt, Baker said.

Baker was to formally present the plan in his speech to the international lending institutions later today, but unveiled the broad outlines in a morning briefing.

Meanwhile, World Bank President A. W. Clausen announced today that he would step down after the conclusion of his five-year term that ends in mid-1986. Clausen, a frequent target of criticism from U.S. officials, told the annual meeting that his speech today would be "my last address to you, the board of governors, as your president." He said that his original intention had been to serve a single five-year term.

Speculation on a successor to Clausen has focused on Federal Reserve Board Chairman Paul A. Volcker.

Baker's proposal reflects a major policy shift that has occurred in recent weeks in the Reagan administration. The administration now apparently believes that the world debt crisis can be solved only if governments and multinational institutions take a more active role. Until recently, the administration had said the debt problem was almost solely an issue between the borrowing countries and their commercial bank lenders.

However, the United States bowed to opposition to parts of the plan from other industrial countries as well as from some of the nations that were supposed to benefit.

Baker conceded that the United States had been forced to put aside -- perhaps because it had been "too innovative" -- one part of his proposal that would have doubled to $5.4 billion a special joint World Bank-IMF fund to ease conditions in countries of sub-Saharan Africa and other weak developing nations. But he said that, in his speech later today, the United States will indicate that it is still interested in a bolder approach along these lines.

He also backed off a proposal to have the World Bank guarantee repayment of some commercial bank loans to developing countries -- a tactic the Treasury had hoped would encourage private banks to begin to lend to Latin America again even though bankers feel they already have made too many loans to the region. He said the United States will continue to explore this concept in later negotiations.

The basic thrust of the Baker initiative, which he said "still needs to be fleshed out," is that the case-by-case debt strategy in force since 1982 must now be broadened by more generous lending to enable borrowing nations to generate the economic activity they need to pay their bills.

Baker ruled out for now, however, an increase in the World Bank's resources that would raise its lending potential. He said a general capital increase for the bank is not needed immediately because there is plenty of room for that agency to pump out more loans using its present resources.

He said that the World Bank, along with the Asian Development Bank, the Inter-American Development Bank and other multinational lending institutions, could push their projected current annual lending rates of about $18 billion to $27 billion in three years.

His proposal for a $20 billion increase in commercial bank lending over the same period would represent a massive increase. Private bank loans to Third World countries last year had dwindled to $17 billion, compared with $48 billion in 1983, according to the IMF.

Baker said that he has "no intention of twisting arms" to persuade commercial banks to boost their new commitments to the debtor nations. But he said "the banks will participate if they see it is in their interest to do so. They have quite a stake now in the Third World , and if I were in their shoes, and if I were convinced that the developing countries could achieve economic growth to pay off their debts, I would do it out of self interest. It depends on whether other elements come together," including loans by the international lending institutions.

Baker defended the U.S. program against criticism from officials such as Brazilian Finance Minister Dilson Funaro, who had labeled the proposal "timid" and said it might have been more useful if launched two years ago.

"I don't think it's too late," Baker said. "If we thought so. we wouldn't be making it."

Baker stressed that the developing countries will need to make major changes in their own economies if they are to attract additional loans from commercial banks. He said it would be unrealistic of them to expect the private banks to send large amounts of fresh money into their countries when there is now a capital flight out of the Third World. He said that others would have no confidence in propects for Third World recovery if their own citizens seemed to lack such confidence.

Initial reactions to Baker's proposals were skimpy in advance of the actual speech. But those who heard his press conference remarks credited him for putting forward some interesting ideas. Some representatives of Third World countries said the test will be in the follow-through.

"To some extent, the United States is suffering from past policies. Baker seems to be trying to turn things around, but we will have to see results," a representative of one country said.

Baker said that the contribution from the borrowing countries would have to be in the form of a commitment "to implement . . . structural policies to improve economic efficiency; mobilize domestic resources; and provide incentives to work, save and invest domestically."

He said that if all of the participants in his proposed strategy do their part and the demand for "quality" World Bank lending increases, the World Bank "should be encouraged to respond effectively and resources should be made availabe to enable it to do so."

But when asked if that meant that the United States, which had been reluctant to endorse a general capital increase for the World Bank, has now changed its mind, Baker said flatly: "No."

He added, however, that if "all of the pieces of the program come together and the others do their part, there will be a need for greater resources, and at that time the United States would look with favor on it , but I don't think we should do it in advance."

By implication, Baker was sharply critical of World Bank performance. He said that if his program were in effect right away, the bank would be making "a lot more" structural and sectoral loans. These types of loans work their way speedily through the economy, in contrast to project loans for buildings and other facilities that may take a much longer time to complete.

Baker also said the bank would be doing more co-financing with commercial lenders, would be reducing the preparatory time it takes to make loans, and would be lifting the ceilings it now places on the amounts of money an individual country may borrow.

"The bank would be a more major player," he said.

Baker originally intended to propose combining IMF and World Bank resources to help the poorest nations in Africa, but backed off in part because the countries that would benefit felt it would have interfered too much in their internal economic policies.

Baker wanted to combine the $2.7 billion that exists in a special IMF fund with a like amount from the World Bank to establish a jointly administered, $5.4 billion trust fund for financially strapped African nations.

Instead, the $2.7 billion IMF trust fund will be left intact in a narrower role -- to come to the aid of the poorest nations when they are having trouble paying their international bills.

Because Baker had not unveiled his plan, the IMF's Interim Committee discussed the debt issue for a day but reportedly failed to take any actions or make recommendations to deal with it. In a communique issued last night, the committee said that "many heavily indebted countries have made considerable progress in their adjustment efforts, but noted that serious difficulties remain."