NEW YORK — MULTINATIONAL companies - the giant firms with plants and offices spread throughout the globe such as NEC of Japan, or General Motors of the United States - like to brag that they are the engines of change for the world economy.Now, a landmark study just released here by the United Nations suggests that the multinationals may be on target with that assessment - and in fact have more clout than they ever imagined. The foreign investments of the global companies have been not only outpacing world trade, but taken together are "increasingly becoming an engine of growth in the world economy," says Peter Hansen, executive director of the UN Centre On Transnational Corporations (CTC), which produced the report. The study, which began in the summer of 1990, is believed to be the first comprehensive examination of the global investments of multinational companies. The UN study team looked at the financial relationships of global companies. The study suggests that there are now three main financial and economic blocks within the world - the European Community, Japan, and the US. One finding was especially stunning: Total foreign investment of multinational companies has been growing three times faster than total world trade since 1983. In the period 1983 to 1989, foreign direct investment grew at a compounded annual rate of 28.9 percent; world exports, by contrast, grew at a compounded annual rate of 9.4 percent. And world gross domestic product - in other words, world output - grew at a much slower pace of 7.8 percent annually. According to Karl Sauvant, acting assistant director of the policy and research branch of CTC, the investments of the transnational companies are increasingly influencing the entire structure of the global economy - from world trade to technology transfers to capital transfers. The capital outflows involved in investments by multinationals are enormous: In 1989, for example, they added up to $196 billion. That year the "total worldwide stock of this [transnational] investment stood at approximately $1.5 trillion," the study concluded. Moreover, "the gap between the growth rate of exports and that of foreign-direct-investment outflows has widened dramatically" since 1985. Factors explaining the sharp jump in direct investments by companies include the strong worldwide recovery following the recessions of the early 1980s; the merger and acquisition fervor of the period; plus corporate efforts to prepare for the economic integration of European Community (EC) nations in 1992. Among other conclusions of the report: * While the global pattern of direct investment was largely bi-polar in the early 1980s (the US and the EC), it was clearly tri-polar by the end of the 1980s, as Japanese companies expanded their operations. * The overall direct investment of global companies is increasingly concentrated in the industrial "Triad": the US, the EC, and Japan. * Japan could have "an equal, if not greater, role in the Triad vis-a-vis the other two members in the near future." * The US role in the Triad shifted during the 1980s; Japanese companies now invest more abroad than US firms; meantime, the US has become the primary recipient nation in terms of receiving direct investment from other Triad members. * Finally, third-world nations find that to obtain foreign direct investment, they must become affiliated with one of the members of the Triad. More than 80 percent of the overall direct investment of the transnational firms is staying within the Triad. The percentage going to developing nations has dropped more than 5 percent during the past decade, and is concentrated in just 10 countries. The policy implications of all this are considerable, suggesting that global trade negotiations need to take far greater account of the role of transnational companies.