After three wars in less than 38 years, India and Pakistan can agree on hardly any major issue these days. Yet at meetings around the world, they both join other Third World nations in castigating industrialized countries, especially the United States, for what they see as protectionist trade policies against their exports.

When it comes to trade between themselves, however, India and Pakistan are fiercely protectionist.

After a round of talks here last month designed to ease strains between the two neighboring nations, India and Pakistan failed to agree on policies that would increase their two-way trade, which has been falling steadily.

Although there is widespread mistrust between the two countries that makes it hard to reach any agreement, spokesmen for India and Pakistan said the snag on the issue of trade is purely economic -- not political.

The difficulties that India and Pakistan have in lowering trade barriers between them are mirrored throughout the developing world, where despite ringing rhetoric over the past 15 years, Third World nations have been unable to come up with a formula that would give their products preferential treatment in each other's markets.

"The diversity is so great among less-developed countries that it is a real challenge to come up with a system that spreads the benefits to a large group of countries. That is the challenge that we are trying to meet," said a top trade official here.

India Commerce Minister Vishwanath Pratap Singh acknowledged in the opening speech to a conference here last month of 71 Third World countries on increasing trade that "progress . . . has been somewhat slow." Prime Minister Rajiv Gandhi blamed that slow pace on "the colonial legacy that most of our countries have inherited," which "has not been conducive to the development of trade amongst ourselves to its full potential.

"It has left us with trade patterns which emphasized trade between developing and industrialized countries, often with the neglect of trade amongst developing countries. During colonial times, links even with close neighbors had weakened and many potentially valuable types of economic exchanges were left unexploited," Gandhi continued.

Nevertheless, officials here said many Third World countries had broken free of colonial trade patterns to forge new economic links with other developing countries.

India, for example, has the potential of joining the most advanced Third World nations as a newly industrialized country and now supplies manufactured products to countries that have not advanced as far as it has.

And the newly industrialized nations of the Pacific Rim have become giants in selling products ranging from consumer electronics to steel and shoes all over the world.

The main barrier to increased trade among Third World countries is "national self-interest," said a long-time western observer of the halting efforts of less-developed nations to engineer greater economic cooperation among themselves.

Although they pay lip service to the concept, only 71 nations attended last month's trade meeting here, even though invitations were sent to all 126 developing countries that belong to the Group of 77 bloc in the United Nations.

And only 28 countries were represented by ministers, which would have given a political push to the issue that cannot come from a lower level of representation.

Most conspicuous by their refusal to send high-level delegations were the ASEAN nations of Southeast Asia -- Indonesia, Malaysia, the Philippines, Singapore and Thailand -- which form one of the most vibrant trading groups in the world.

India reportedly pressed hard to get the ASEAN Bloc to send its trade ministers here, but was rebuffed. "We missed the ASEANs. There is no doubt about it," said an Indian official connected with the Third World trade meeting. He maintained that ASEAN countries would benefit most from any relaxation of trade barriers between less-developed countries.

"They are burgeoning economies; they are export-oriented," he said.

But he acknowledged that India is unlikely to relax its strict protectionist policies, designed to keep foreign competition away from its domestic industries with tariffs as high as 200 percent and stiff licensing requirements for imports, as part of a program to increase trade among Third World nations.

The ASEAN nations, along with other Third World states that appear unsure how this concept of "a global system of trade preferences" for the less-developed countries would help them, continue to go along with the idea.

One reason is political -- Third World solidarity. Another is the feeling that if the Third World could coalesce in the economic sphere, they could gain the same level of political clout in organizations such as the World Bank or General Agreement on Tariffs and Trade (GATT) that their numbers give them in the U.N. General Assembly.

The problem is that most Third World countries have the same products to sell -- textiles, processed tropical products and agricultural commodities -- which makes it hard to set up a trade relationship.

Some of the mid-level Third World countries such as India and Brazil see great economic gains from an increase in trade between less-developed nations. They see themselves replacing the industrialized West in supplying some less-sophisticated products to Third World countries that have not achieved a level of manufacturing beyond textiles.

But Third World economists acknowledge the real problem is how to spread the benefits to the poorest of the poor -- who have little to sell to anyone, and especially to countries that are only at a slightly higher stage of development than they are.

There have been increases in the trade among Third World nations over the past 15 years, mostly because of oil sales by petroleum-producing countries.

According to statistics compiled by the Research Center for Cooperation With Developing Countries in Ljubljana, Yugoslavia, trade among developing countries increased from $11 billion in 1970 to $145.6 billion in 1982.

Trade in manufactured goods -- up from $3 billion in 1970 to $43 billion in 1982 -- amounted to about one-third of the commerce between Third World nations.

Now the question is whether this trade will be accelerated.

The New Delhi meeting tried to set it on a fast track, with negotiations to start next May 1 and to be completed in a year. The aim is to get an across-the-board 10 percent tariff cut -- a drop in the bucket considering the astronomically protectionist tariffs that most Third World nations impose on manufactured goods -- as well as easied restrictions on specific products of importance to the least developed countries.

Brazil sent its foreign minister, Clavo Egydio Setubal, here to invite the Third World nations to have their next meeting in his country.

But it may prove easier to set up goals and schedule new meetings than come to concrete understandings that will lead to eased trade restrictions.

It is far easier, moreover, for Third World nations to come together to attack trade restrictions by the industrialized nations in areas such as textiles than to agree to lower their own barriers.

Despite the promises of Gandhi to liberalize India's economy, for example, India this month turned down a proposal by PepsiCo Inc. that would have provided new technology for processing fruits for overseas sales in exchange for allowing the company's soft drink into this country.

Pepsi estimated India would receive $3 in foreign exchange from increased exports for every $1 it spent to buy the Pepsi concentrate.

According to widespread reports here, the deal was turned down by the Gandhi government because of pressure from India's own manufacturers of soft drinks, who feared they would be driven out of the market if Pepsi was allowed to sell here. "In the world of free trade, where the United States likes to think itself a leader, no one is 100 percent pure," said Ken George, director of the Commerce Department's U.S. and Foreign Commerce Service during a visit here last week.

"I would think that would apply to India's position on Pepsi and our position on textiles" -- placing tighter restrictions on imports, he added.

"We live with political realities," he said.