The Department of Health and Human Services announced yesterday that it had granted approval to three companies to market much cheaper copies of Valium, a well-known tranquilizer that has long been one of the country's top-selling prescription drugs.

The move is part of a campaign to make less costly drugs available, and followed enactment of a law making it easier to get government approval for generic drugs. Valium copies may cost less than half the price of Valium, generic manufacturers said yesterday as they rushed their drugs to market.

Valium is used by an estimated 8 million to 9 million Americans each year for relief of tension, anxiety and other disorders, and the market for the drug, known chemically as diazepam, had been held exclusively for more than two decades by Hoffmann-La Roche Inc. of Nutley, N.J.

The company's patent on the drug ran out in February.

In a strong endorsement of more widespread use of generic drugs, HHS Secretary Margaret M. Heckler said that, with the new approvals, "American consumers can now purchase at significant savings alternative versions of nine of the top 10 drugs used in the United States."

Four of the top 10 sellers have been approved for generic manufacture since June under the simplified procedures of the new act: Valium; Inderal (chemically called propranolol hydrochloride) for heart and blood pressure problems; Darvocet-N100 (a version of acetaminophen) for pain; Motrin (ibuprofen) for pain and arthritis.

"Generic versions of drugs do exactly what the 'brand name' drugs do, although generics may vary from the brand names in taste, appearance or in inert ingredients," Heckler said.

"But the consumers in our country who now take the leading prescription painkillers, the leading arthritis drug, the top high-blood-pressure drug and the top-selling tranquilizers can now save money by asking their physicians and pharmacists to prescribe and dispense generic versions of these products."

She urged consumers to ask, "Is there a generic equivalent I can get at a lower cost?"

It is uncertain whether the new generic products will cut into the existing Valium market or increase overall use because of the cheaper prices.

Valium was the No. 4 drug nationally in terms of sales last year. In 1975, it was the No. 1 prescription drug, with 60 million prescriptions annually. Sales have declined because other tranquilizers have entered the market, and because of some concern about overuse or addiction.

HHS listed last year's Valium sales at more than $350 million, while Hoffmann-La Roche Inc. cited lower figures of $240 million in 1984, with 25 million prescriptions for the tablet form.

Hoffmann-La Roche said it is not planning to increase the price of Valium.

"We just don't know what the impact of this will be on the marketplace," said Dr. Bruce Medd of Roche Laboratories, the pharmaceutical division of Hoffmann-La Roche. "I certainly can't say it will have a positive effect."

Prices vary, but the Association for Retired Persons lists 100 5-milligram tablets of Valium at about $21.50.

The House Energy and Commerce subcommittee on health and the environment, which conducted a study of prescription drug price increases, said the cost of Valium rose 81 percent between Jan. 1, 1981, and July 1, 1985, compared to a rise of 23.7 percent in the Consumer Price Index.

Hoffmann-La Roche said their pricing reflected the costs of researching new drugs.

The subcommittee counsel William V. Corr said yesterday's action was a deliberate decision by the government to approve the manufacturing of diazepam by three companies at the same time. "It will promote competition," Corr said. "I would expect to see relatively aggressive pricing strategies."

The Drug Price Competition and Patent Term Restoration Act took effect last November, removing barriers to getting generic drugs on the market and extending the life of some manufacturers' patents on newer drugs by up to five years.

The law has particularly helped firms copying brand-name drugs approved since l962, when the federal law began to require extensive proof of safety and effectiveness. The proof involves lengthy, expensive tests in animals and humans.

Until the new price competition act, firms wanting to copy drugs had to repeat the extensive testing.

Now they can use the earlier studies, and try to prove to the Food and Drug Administration "that they actually effect the body in the same way the original did," agency spokesman William Grigg said.Heckler said that "generic drugs are subject to the same stringent FDA standards as the brand names," but companies such as Hoffmann-La Roche have challenged this contention.

Roche Laboratories' Medd said yesterday that "we still have concern about bioequivalency" of generic versions of Valium. He said generic versions produced overseas acted much more slowly on the brain. But he added that "we don't know what the quality of these diazepams being approved today is." Yesterday's FDA approval went to Mylan Pharmaceuticals of Morgantown, W.Va.; Parke-Davis Division of Warner Lambert Company of Morris Plains, N.J.; and Zenith Laboratories of Northvale, N.J.

Spokesmen for all three companies said their versions of Valium would be sent to wholesalers immediately and would be available soon to patients with prescriptions.