When Indochina fell in the spring of 1975, a wave of worry was concentrated on the nearby states of Southeast Asia. This week, three years later, the victorious communist regimes are warring and are impoverished and the foreign ministers of the potential Southeast Asian dominos, still independent and thriving, are coming to Washington to forge a new relationship with the United States.

Unlike the Johnson-Nixon era meetings with Asian leaders, the conference starting Thursday morning at the State Department will center on trade other economic matters rather than security concerns. With the participants of President Carter and five members of his Cabinet, the two days of meetings will provide a symbolic new start for the United States in the region of its greatest ravail and most humiliating defeat.

The Association of Southeast Asian Nations (ASEAN) - Indonesia, Malaysia, the Philippines, Singapore and Thailand - was founded in 1967 but did not amount to much until the U.S. withdrawal in Indochina left these countries to their own devices and concentrated the minds of their leaders on cooperative arrangements.

Until recently, the communist powers in Asia condemned ASEAN as a puerile tool of imperialism. The United States, on the advice of senior career diplomats, ignored it as a "1/2nonevent." Now attitudes have shifted all around.

Both China and the Soviet Union have swallowed their opposition and are beginning to court the Southeast Asia grouping and its members. Vietnamese Vice Foreign Minister Phan Hien said early last week during a swing through Southeast Asia that "we are prepared to hold discussions with each of the ASEAN member countries and the organization as a whole." He did not rule out the possibility that Vietnam eventually might join ASEAN in some capacity.

In recent months ASEAN as a regional grouping arranged and carried out consultations, one after another, with Japan, Canada, the European Economic Community, and Australia-New Zealand. Building on a lower-level meeting with a U.S. team last September in Manila, the Washington session this week is seen as the capstone on ASEAN's international recognition.

"We want the Americans to see us collectively and to see the problems we all have, and to appreciate that economic stability is fundamental to political stability," said an Asian diplomat here who has been involved in preparations for the sessions. He said ASEAN leaders are prepared to ask for specific commitments from the United States on such matters as tariffs, investment and aid to the refugees from Indochina who have swarmed into several of the countries. Equally important, he added, will be the establishment of a general framework for U.S. involvement with the region and its grouping.

U.S. officials are not ready to approve the special trade preferences which ASEAN has requested, but there are plans to announce missions to the area by officials in the investment guarantee field. Also planned is discussion of cooperative arrangements on energy, food, science and technology and a small amount of aid to ASEAN as an organization.

"This will be the first time the U.S. government has tried to come to terms with Southeast Asia as a region since the fall of Saigon," said a senior U.S. diplomat involved in this week's sessions. "It's important that the participation is at an appropriately high level, and it's important for Americans to begin to see something in Southeast Asia as positive after 20 years of overcommitment in the area and three years of post-Vietnam trauma."

The combined population of the five ASEAN countries is 245 million, about the same as the United States and Canada combined. Their rubber, tin and other resources, including oil and gas in the case of Indonesia, as well as their geographic position on the sea lanes between the Middle East and Japan add up to a position of importance.

As U.S. diplomats see it, economic progress and security in the ASEAN area can be a modest plus for Asia as a whole. And economic failure, political instability or the installation of hostile governments in the area could be a setback of serious importance.

ASEAN is not a military pact or security organization, and its sponsorship of a Southeast Asia "zone of peace, freedom and neutrality" has given the organization a somewhat neutralist image. The Philippines, however, is the site of major U.S. bases at Clark Field and Subic Bay which Manila expects to retain under renegotiated arrangements with the United States, and most of the ASEAN countries have made clear their desire for a continued U.S. military commitment to the Pacific.

"We do have problems and we are a varied group of countries, which don't see everything the same way," said an ASEAN nation ambassador here. "But we'd like to look after ourselves, with some help from the big brother we prefer, even if far away, rather than the big brothers we don't want." He said this week's meetings will be a test of the preferred "big brother" and his attitudes three years after the war most Americans want to forget.