Small amounts of suspected cancer-causing chemicals known as PBBs have been found around three manufacturing plants near New York City, the Environmental Protection Agency announced yesterday.

It was the first three finding of traces of the chemical in this country since a widely publicized Michigan case in 1973 where PBBs were mistakenly mixed with animal feed. Tens of thousands of animals had to be destroyed after the discovery and there were signs of human illness, too.

EPA officials said yesterday that the latest discovery was not cause for alarm. But they added that tests will begin immediately on the milk of nursing mothers in the northern New Jersey and Staten Island area to determine the extent of possible human exposure.

The chemcial, polybrominated biphenyls, an industrial flame retardant, has been found to cause cancer in laboratory animals. It was therefore one of 15 chemicals picked for priority EPA study earlier this year when the Toxic Substances Control Act went into effect.

The search for PBBs turned up residues in samples of human hair, in fish, soil, water and plants near the White Chemical Co. plant in Bayonne, N.J., and the Hexcel Corp. plant in nearby Sayreville. Both companies manufactured PBBs for export to Europe.

Smaller traces of the toxic chemical were discovered in similar samples at the Standard T chemical plant in Staten Island, N.Y., which stopped using PBBs to make wire coatings a few years ago.

"People won't be dropping dead from this stuff," William Coniglio, EPA office of toxic substances coordiantor for the PBB tests, said yesterday. "But we really aren't sure of the scope of this problem. We need more tests to see how extensive and intense it is."

A spokesman for the Hexcel Corp. plant said yesterday that his company disputed EPA's contention that it made PBBs. He said the firm made only small amounts of a related but much less toxic chemical over the past few years for selected customers in Europe.