The Treasury Department yesterday said it would brings its first formal action against foreign steel companies who are selling their products below the minimum import prices established by the government as part of its program to protect the domestic steel industry.

Treasury officials said the long-awaited charges would be brought against three steel companies: one from Poland, one from Spain and the third from Taiwan.

They said the three companies have been selling "significant quantities" of plate steel at prices below the minimum, or "trigger," prices tht the Treasury has established for the product.

The Treasury said it has reason to believe that these sales are also at less than fair value - in the case of steel that usually means below the cost of production - and are causing injury on U.S. steel makers.

Since the trigger price mechanism went into full effect on May 1, the Treasury has carried out more than 200 investigations into foreign steel sales that were made below trigger prices, but officials said in each of those were not being made below fair value.

Selling products below fair value is called dumpling and is a violatin of U.S. trade laws.

During the four months from May through Aug. 31, about 917,000 tons of plate steel were imported into the United States. The three countries involved accounted for about 228,000 tons of that.

Officials said yesterday that about 25 percent of the imports from Portland, Spain and Taiwan were sold below trigger prices by the three companies in question, or about 57,000 tons of the seel product. At a rough value of $400 ton for the product - which is used in heavy projects such as bridges or ship hulls - the amounts involved total about $23 million.

Imports of plate during the first eight months of this year accounted for about 24 percent of total domestic use, up sharply from the 18.5 percent share of the market plate imports accounted for in the first eight months of 1977.

One of the reasons the Treasury waited so long to bring formal anti-dumping charges under the trigger price procedure was "our desire to have a clean case, a strong one," an official said.

In a complaint to be published in the Federal Register next Tuesday, the Treasury alleged that the sales occured as much as 48.5 percent below cost.

The Treasury has given the three companies involved 21 days to supply price information, including what the products sold for in their home market, and 35 days to supply cost-of-production data. If the companies do not supply the information by the deadline, the agency said it would use the best evidence it has to calculate the fair value of the products involved.

The Treasury set trigger prices for steel products based on the cost of production in Japan, reportedly the world's most efficient producer, plus the cost of shipping to the United States.

If products come in below trigger price and the exporter cannot justify the price (in many cases the imports are to companies related to the foreign company and the first sale to an unrelated company in the United States is at a price above the trigger price), the Treasury is supposed to launch a speeded-up anti-dumping investigation.

If it is determined that dumpling exists, and Treasury officials think that in the cases announced yesterday there is dumping, then special duties are levied.

Despite the existence of the trigger price mechanism, which in effect sets a floor on both domestic and imported steel prices, steel imports into the United States surged in July and August, after subsiding during the first two months of the program. Many of the imports came from countries such as Taiwan and Spain as well as from Europe.

Japan, which had been a big exporter of steel to the United States, has held down on its shipment because of official U.S. displeasure with the large amount of foreign steel being sold in the United States.

The companies involved in yesterday's investigation are: Empress Nacional Siderurgica of Spain; Stahlexport Przedsiebiorstwoa of Poland; and China Steel Corp. of Taiwan.