The world's industrial nations will enjoy a 3 percent real growth rate this year, breaking out of a long period of economic stagnation, according to a highly optimistic appraisal by International Monetary Fund Managing Director Jacques de Larosiere.

In a speech prepared for delivery this morning to the United Nations Conference on Trade and Development in Belgrade, de Larosiere said that the improved "momentum" will continue into next year, triggering an upturn in world trade, and relieving worries over the Third World debt problem.

A 3 percent growth rate this year would compare with a 0.3 percent rate in 1982. If a 3 percent rate is sustained "over the next few years" and the borrowing nations put their own houses in order, de Larosiere said, "the financial problems of the developing countries are manageable."

De Larosiere's speech, which was released here by the IMF, was the most bullish assessment of worldwide economic prospects by any official in recent months.

It is based on IMF staff projections of an ongoing improvement in four leading nations--the United States, United Kingdom, Japan and West Germany--that together account for about half of world production. Sources suggested that the 3 percent figure used by de Larosiere may be on the conservative side, implying that growth during 1984 will be even better.

At the Toronto annual meeting in September, 1982, the IMF estimated 1983 economic growth in the industrial world at 2.5 percent. But that was based on expectations of a stronger first half than actually developed.

De Larosiere told his UNCTAD audience that less developed countries, helped by the industrial recovery and an upturn in some commodity prices, would enjoy about a $20 billion annual gain in their merchandise export earnings.

But de Larosiere also subtly rejected Third World demands, echoed by some in Congress, that the IMF sell off a portion of its gold. He said that to preserve its financial strength, the IMF must maintain "the integrity of its assets."

Recent economic activity, especially in the United States, appears to have improved. At the Williamsburg economic summit, Treasury Secretary Donald T. Regan predicted that the second quarter growth rate would jump to a 5 to 6 percent range from 2.5 per cent in the first quarter. He also predicted a 6 percent real gain, fourth quarter 1983 over fourth quarter 1982.

With the impetus of greater activity in the industrial world, growth in the developing countries (excluding oil exporters) would "pick up somewhat to about 2.5 percent" from 2 percent last year, de Larosiere predicted. At the same time, their deficit on current account (covering goods and services) would decline to less than $70 billion, or down significantly from $87 billion in 1982, and some $40 billion from the peak $110 billion deficit in 1981.

The IMF executive also said that despite the "welcome and encouraging developments" relating to economic recovery, the underlying situation "contains a number of weaknesses and uncertainties." Chief among these are intensified protectionist pressures; excessive reliance on monetary policy in many countries; large budget deficits; and the buildup in Third World debt to a total of $600 billion at the end of 1982.