The Environmental Protection Agency yesterday extended water pollution controls to three largely unregulated industries in hope of stopping yearly discharges of 12,000 tons of toxic wastes into natural bodies of water and sewage treatment plants.

The new restrictions issued for 63 toxic pollutants discharged by the plastics, synthetic-fiber and organic-chemical industries were described by Lawrence J. Jensen, EPA assistant administrator for water, as among "the most ambitious and important water pollution control regulations ever issued" by the agency.

An EPA spokesman said the three industries, consisting of nearly 1,000 plants with annual revenues of $59 billion, represent the nation's largest source of water pollution.

The courts struck down EPA controls for organic chemicals in the 1970's, prompting the agency to withdraw proposed regulations for the plastics and synthetic-fibers industries, which were similar to those for organic chemicals. The EPA has since been mired in bureaucracy, blocking new toxic waste standards, according to the spokesman.

Yesterday's regulations apply to discharges of conventional as well as toxic wastes. The three industries have been subject to case-by-case controls on conventional wastes, such as plastic resin and dissolved salt.

The standards unveiled by Jensen are to be imposed uniformly on all plastics, organic-chemical and synthetic-fiber plants. They are expected annually to stop discharges of 54,000 tons of conventional wastes, which deplete oxygen in the water.

Although past controls on conventional wastes coincidentally trapped some hazardous substances, the new regulations are the first to directly regulate such toxic pollutants as benzene, which causes leukemia, and lead, believed responsible for neurological disorders and obstetric complications.

Costs of complying with the new standards are expected to reach $500 million a year, according to the EPA, enough to force as many as 61 plants employing 3,400 workers to close. The three industries employ 183,000 people.

The regulations require plants to use the best available technology for control of toxic pollutants. To soften the economic impact on smaller facilities, however, the agency will accept the best technology affordable.

Environmental benefits from the controls, including better fishing, are said to be worth at least $178 million per year.