The International Trade Commission found unanimously yesterday that the three largest semiconductor-chip makers in Silicon Valley were injured by low-price competition from Japan.

The preliminary ruling by the five-member commission was the first step in a long governmental process to determine whether the Japanese have been selling computer chips at prices below their manufacturing costs in an effort to capture a large share of the U.S. market.

If the Commerce Department finds that the Japanese-made EPROM erasable programable read-only memory chips have been dumped illegally in this country, penalty duties will be assessed against them. These duties could amount, in some cases, to more than the chips are being sold for today, if the charges by the American companies are upheld. The three U.S. companies involved in the complaint are Advanced Micro Devises Inc., Intel Corp. and National Semiconductor Corp.

This case is one of several filed this fall by the financially battered U.S. semiconductor industry, which has been hit hard by an unexpected slowdown in computer sales and increased competition here and in overseas markets from aggressive Japanese manufacturers.

Sources said preliminary talks have begun toward a possible settlement in the most far-reaching case. That case involves a complaint by the Semiconductor Industry Association that U.S. makers have not been given a fair shake in the Japanese market over a long period.

Deputy U.S. Trade Representative Michael Smith is scheduled to hold talks on the industry complaints with his Japanese counterparts from the Ministry of International Trade and Industry in Tokyo in 10 days. These talks were described by industry sources as preliminary, however, and U.S. Trade Representative Clayton Yeutter told a Senate committee last month that the issue was too important to be papered over with a market-sharing settlement.

The ruling yesterday by the ITC was filed Sept. 30 against eight companies, including some of Japan's high-technology giants: Fujitsu Ltd., Hitachi Ltd., Matsushita Electric Corp., Mitsubishi Corp., NEC Corp., Oki Lit., Ricoh Corp. and Toshiba Ltd. They all denied the allegations in the complaint.

Intel's chief counsel, F. Thomas Dunlap, said the Japanese companies are practicing "predatory" pricing to gain a share of the U.S. market.

The U.S. makers charged in their complaint that an EPROM priced at $17 in January, when Japan aggressively entered the market, sold for only $4 in August. The U.S. companies estimated that it cost their Japanese rivals $6.34 to make the chip.

The ITC staff reported that a "sharp decline" in EPROM prices began in the middle of 1984, and by October had fallen as low as 10 percent of the earlier price.

A knowledgeable observer said the American firms were caught in a squeeze by their Japanese competitors, either having to drop prices below their costs to maintain their share of the market or keep their higher prices and lose sales.

ITC Chairman Paula Stern said the case concerns "the impact of imports on the economic health of a high-tech, multinational industry" that ships labor-intensive jobs overseas while retaining research and design and fabrication activities in this country. "The corporations within this industry base their production decisions on worldwide economies of scale, balancing enormous capital investment costs with relatively cheap labor rates."

For its final determination, the ITC must examine costs in each production step, wherever they occur, to determine how the law "applies to rapidly evolving, global, high-technology industries," she said.