WASHINGTON (AP) _ Three senators told a top trade official from Japan on Friday that if Japanese carmakers don't buy more U.S. auto parts, Congress may pass protectionist legislation the Japanese would regret.

''If we don't have tangible results to show there are changes ... you're going to get legislation that would certainly be harmful to Japan but also, if it goes too far, could be very counterproductive to this country,'' said Sen. Dan Quayle, R-Ind.

U.S. manufacturers supply less than 1 percent of the $55 billion worth of parts and components used in cars made in Japan, and only about 40 percent of the parts used in cars made at Japanese plants in the United States.

Quayle and Sens. Carl Levin, D-Mich., and Pete Wilson, R-Calif., met with Makoto Kuroda, vice minister of Japan's Ministry of International Trade and Industry, at the end of the latest round of bilateral negotiations on U.S.-Japan trade in auto parts and accessories.

Joining the senators was Bruce Smart, a Commerce Department undersecretary who is leading the U.S. delegation in the so-called MOSS talks, short for market-oriented, sector-specific negotiations. The talks on auto parts began last August and are expected to last about a year.

Smart said that while the Reagan administration doesn't endorse protectionist legislation to address the auto parts trade imbalance, pressure from Congress provides the U.S. negotiators with leverage at the bargaining table.

Smart said ''modest progress'' has been made thus far in at least persuading the Japanese to provide more information about their auto parts market. He said the talks are unique in that they focus not on persuading Japan to remove government-imposed barriers to trade, but on the buying habits of individual Japanese carmakers who have longstanding relationships with local suppliers.

''What we would like to see is that American companies can compete with Japanese companies based on quality, service and price and not on old-time, old-boy networks or on national favoritism,'' Smart said. ''We don't think they have the opportunity now.''

Smart said that while it is ultimately up to the individual carmakers to decide from whom they will purchase their parts, ''there is clearly an opportunity for the Japanese government to be influential in this issue.''

Added Quayle: ''The Japanese government can't just passively sit back and not pay attention to a $50 billion trade deficit. ... They can create an environment to encourage changes in business practices.''

Kuroda commented only briefly as he left the meeting, saying that work on the auto parts issue has only recently started, and ''I am expecting good progress will be achieved in the future.''