For the third time since August, the Commerce Department ruled yesterday that Japanese companies violated international trade laws by failing to increase their prices to match the sharp rise in the value of the yen in the past two years.

The ruling yesterday was against seven Japanese manufacturers of forklift trucks and follows findings last week against manufacturers of color picture tubes and in August against makers of roller bearings.

These decisions are seen as precedents that will be used by American companies that have not regained domestic sales from Japanese competitors despite a 50 percent increase in the value of the yen since September 1985.

"We are looking into possible dumping in a variety of products," Commerce Secretary C. William Verity said in a speech prepared for delivery today to the Japan National Press Club.

Verity has warned Japanese companies during his visit to Tokyo that their pricing practices are distorting world markets and undercutting international efforts to help ease the United States' record trade deficits through currency adjustments.

"By failing to price in line with the appreciated yen in order to maintain market share, Japanese firms are impeding the power of currency adjustments to help correct our trade imbalance," he said in another speech prepared for delivery today, this one to the American Chamber of Commerce in Japan.

Verity said Japanese export prices have declined 23 percent and now are as much as 10 percent below wholesale prices.

"What this means is that Japanese exporters are trying to save market share by maintaining a constant or even {a} declining dollar price," the Commerce secretary said. "We estimate that price cuts by Japanese exporters have offset half the increase in the value of the yen since January 1985."

Verity's comments thus took the concerns of American manufacturers to a higher political level, with the Reagan administration taking a position on the need to drive up Japanese prices, and added one more irritant to U.S.-Japanese trade relations.

Verity's visit to Japan, the first by a Reagan administration Cabinet member since Noboro Takeshita took over as prime minister two weeks ago, has focused on trade despite efforts to paint it in broader terms of the trans-Pacific relationship.

Verity struck out on one key trade issue, the participation of U.S. construction and engineering companies in $62 billion worth of Japanese public works projects, when he was told yesterday that the question should be taken up in global trade talks since other countries -- especially the Europeans -- restrict foreign participation in their public work projects.

Aides traveling with the commerce secretary said Verity told the Japanese their position was unacceptable to the United States.

Verity also said Japanese companies win contracts for U.S. public works projects, showing that Europe's practices are not this country's.

Administration trade officials here saw the Japanese position as a retreat from specific commitments and said government retaliation might follow.

One official predicted that a decision by President Reagan to institute an unfair trade case against Japan could be made within 10 days, although there were reports that Secretary of State George Shultz, who opposes retaliation, would try to postpone it until Takeshita makes his first visit to the United States as prime minister in January.

The issue of allowing foreign participation in public works projects, which came to a head over plans to build the $8 billion Kansai International Airport at Osaka, is a ticklish one in Japan because much of Takeshita's political support comes from Japanese construction companies.

U.S. trade officials, who have been holding negotiations with Japan on that issue for more than a year, are convinced that all major contracts for the airport and for a project to build a $6 billion bridge over Tokyo Bay have been let secretly to Japanese companies. So they want any agreements reached on those two projects to cover $48 billion in other jobs that are under consideration for the next 10 years.

The Takeshita government has refused to go that far, despite a commitment to President Reagan in September by Prime Minister Yasuhiro Nakasone, administration officials said.