For the third time in 18 months, an American president soon will leave the confines of the continental United States, and fly to a summit meeting with other heads of state to discuss economic issues.

This will be the Downing Street Summit, to be held May 7 and 8 in the Georgian office-residence of Prime Minister James Callaghan of Great Britain. In addition to President Carter, the other participants will be Chancellor Helmut Schmidt of West Germany, Premier Takeo Fukuda of Japan, President Valery Giscard d'Estaing of France, Premier Giulio Andreotti of Italy, and Premier Pierre Trudeau of Canada.

Also in attendance, for the first time, will be Roy Jenkins, president of the European Common Market Commission.

Most of these officials are well known to each other. But President Carter, who will be venturing abroad for the first time, never has met Schmidt, Giscard d'Estaing, or Andreotti, and has had only brief meetings on separate occasions wiht Callaghan, Fukuda, Trudeau and Jenkins.

It will be his first experience in a major international meeting of any kind since he became President, and the Europeans - to whom he is still much of a mystery - will examine closely every word he utters in this forum, and in a North Atlantic Treaty Organization meeting that follows immediately.

Advocates of economic summitry insist that the form is as important as the substance at these meetings. There is an almost unique opportunity for heads of state to get to know each other in virtually private sessions.

The first two summits - at Rambouillet, France, in November 1975, and Dorado Beach, Puerto Rico, in April 1976 - completely insulated the leaders from the press. An effort will be made to duplicate this situation in London.

In essence, calling an economic summit meeting is acknowledgement that important international economic problems exist and must be solved. But there is no guarantee that solutions will be forthcoming, a fact of life that sends chills up and down the backs of professional civil servants who try to plot out the agenda and eventual communique or declaration.

Moreover, this summit occurs at a time when European leaders are afflicted with shaky and unpopular governments. And as former U.S. Ambassador to the Common Market J. Robert Schaetzel pointed out last week, "The opposition parties are hardly more popular."

What's more, the European economic outlook is anything but promising, the energy problem is ever-present, and the gap between the strength of West Germany and the Netherlands and the weakness almost everywhere else is growing.

As if that were not enough, sentiment for trade protectionism is growing everywhere, although formally resisted by individual governments and by the Jenkins commission in Brussels.

Plainly, the Europeans plan to use the London summit to prod Japan into more purchases of Common Market goods to trim the fat Japanese trade surplus. Because the United States also is leaning on the Japanese to take voluntary measures to cut their shoe exports to this country, and is likely to follow the same approach on electronic goods such as color TVs, the Japanese may come to think of the Downing Street Summit as a convenient battering ram against them.

In Tokyo, government officials say they are ready to make concessions. But they are alarmed by what they consider a drift to protectionism in Europe, and are fearful that the unions in the United States - unable to win approval of other parts of their program - will help push Carter away from his stated free-trade principles.

In this context, the initial agenda carved out for the summit meeting revolved mainly about two points: One how to spur worldwide recovery by placing greater "responsibility" on the big three industrial countries - the United States, West Germany, and Japan. And, Two, how to satisfy the legitimate demands of the poor countries for more help from the rich.

Within the confines of these two majors topics fall all the other collateral issues - trade, energy, aid, and financing of the poor countries' debt.

For example, "responsibilities" of the big three, as President Carter's economic advisers saw it, included an extra dose of expansion so that more active domestic economies would soak up a greater volume of imports from the poor nations. At the same time, the United States said, West Germany and Japan should begin to run current account deficits as, indeed, the United States already is doing.

That would alter the current picture, which shows the Organization of Petroleum Exporting Countries running an annual surplus of about $40 billion, and major industrial nations (except for the U.S.) running a surplus of about $15 billion. In effect, it would mean - if Germany and Japan joined this country on the deficit side of the ledger - that smaller oil-importing countries could enjoy a surplus or a reduction in their deficits for a change.

But the U.S. push for expansion appears to have come a cropper. Chancellor Schmidt has told American officials that the Germans are doing enough on the side of economic expansion, and to do more would threaten their own stability and the stability of neighboring nations.

The Germans argue that they have allowed the deutschemark to appreciate by 13 per cent in the last year, "a defacto contribution to the competitiveness of other nations." Even so, a high International Monetary Fund official warned last week that his agency might press the Germans and the Japanese for additional appreciation of their currencies.

But the coup de grace to the U.S. push for extra expansion was self-inflicted: Carter decided to abandon all but $3.5 billion of this year's economic stimulus package in the face of a possible defeat in Congress.

Carter will go to the Downing Street Summit arguing that despite the truncated stimulus program, the U.S. growth rate this year still will meet his target of 6 per cent, fourth-quarter 1977 over fourth-quarter 1976. But he clearly is in a weaker position to pressure West Germany and Japan.

President Giscard d'Estaing, despite his shrinking mandate and a weak economy, will try to use the summit, as he has before, to portray France as a strong power. He still will do what he can to push for economic expansion - especially by Germany - and also for a firm commitment by the industrial nations to help the poor ones.

An effort to find a common ground among the summit powers that can renew the stalled North-South dialogue on aid issues may emerge as the centerpiece of the summit now that a coordinated worldwide expansion is being put more or less aside.

West Germany officials say what they want most from the summit is progress on North-South issues. Like other European nations, the Germans are highly dependent on imported raw materials. They would like the United States - which under the Ford administration took a hard-nosed ideological stance against most of the demands by poor nations - to agree to stabilize raw materials earnings in the less developed countries, not prices.

That, the Germans say would not interfere with free market mechanisms. For example, if Sri Lanka were to lose money on one crop, that specific loss would not be made up. But there would be compensatory financing of a shortfall in Sri Lanka's overall average export earnings.

The Germans favor setting up a special financing facility, supported half through capital market borrowings and half by governments (perhaps through IMF administrative auspices.)

The Europeans are anxious to improve the lot of the poor countries not for altruistic reasons, but because they see the Third World as the last, unexplored, undeveloped market for western industrial goods.

The poor nations' proposal at Nairobi last year was for a common fund with an initial stake of $3 billion (ultimately, $6 billion) that would finance a whole range of buffer stocks.American officials remain adamantly opposed to this proposal, but a Carter administration source agrees that the United States is coming close to something like the European approach.

"We might agree that four or five commodities need stabilization, and that it might increase efficiency if there could be cross-lending," he said.

In an interview last week in Washington, U.K. Minister for Overseas Development Judith Hart warned that the poor nations "have decided a Common Fund is much the most important element (in North-South discussions.) It has a symbolic importance, a test of good intentions (on the part of the rich nations)."

Mrs. Hart, on her way back from a conference in Fiji concerning European community relationships with the poor nations, agreed with Jenkins that the Carter administration would be more open-minded that the preceding Republican administration on these issues.Jenkins predicted flatly that a more forthcoming U.S. - European attitude on the common fund question - even if not satisfying all of the poor nations' demands - would emerge as the only substantive agreement of the Downing Street Summit.

Non-economic issues will be brought up as well - that's the reason that the work "economic" was dropped out of the title of this summit. Callaghan told reporters in London that he aims to get Carter's campaign against breaches of human rights on the agenda when East-West relations come up. Callaghan doesn't expect to change Carter's mind, but wants to give him a better perspective of European misgivings on this subject.

On the question of linking human rights to development aid. Mrs. Hart (who was responsible a few years back for cutting off British aid to Chile) takes a practical point of view. She feels there are circumstances where aid should continue to be provided to countries denying human rights, "where such aid might be the only condition leading to a change in government."

When you come right down to it, she says, "the most basic right is to survive and be fed." With that as a guide, the British aid minister feels that it may be better to supply aid, "along with some arm-twisting," rather than cutting it off completely.