WASHINGTON — Most population growth between now and the 21st century will be concentrated in already overcrowded Third World cities, says a new report by the Population Reference Bureau, a research and educational organization. ``Of the 20 most populous metropolitan areas in 1950, only nine were in developing countries. By the year 2000, 17 of the top 20 metropolitan areas are projected to be in developing countries,'' the PRB says in ``America in the 21st Century: A Global Perspective.'' The report says that the expanding city populations represent enormous potential markets for US consumer goods and services if trade channels are kept open and the United States is competitive. Already, one-third of all US exports go to developing countries.