The U.S. current account deficit, the broadest measure of the nation's international trade and financial transactions, dipped slightly during the first three months of the year, the Commerce Department reported yesterday.

The government reported the current account deficit fell to $37.1 billion from $38 billion in the final three months of last year. The report was released as President Reagan told Senate Republicans at lunch yesterday "I would have no choice" but to veto trade legislation if the Senate fails to change a bill passed by the House in April.

Reagan singled out provisions in the House-passed bill that would mandate retaliation. He said such provisions "could well plunge us into a trade war that would very quickly spiral out of control."

"Similarly dangerous provisions are proposed in trade legislation that will be coming to the Senate floor soon," Reagan continued. "We've been working very closely with Senate committees to come up with a bill that would enhance prospects for U.S. exports without risking a trade war."

Reagan called for the elimination of "the worst provisions" of both the House and Senate bills. Otherwise, he said, "I'm afraid the prospects won't be very good for my signing the final product into law."

The Senate Democratic leadership, unable to end a filibuster over the campaign financing bill, yesterday delayed consideration of a trade bill until next week.

Reagan, in his appearance at the Republican leadership lunch, took a low key approach in his criticism of the trade legislation. As part of an understanding between the White House and congressional leaders from both parties, the president avoided the harshly confrontational tone that he used last year when trying to influence congressional trade action and did not use the trigger word "protectionist," to which lawmakers have objected.

His campaign to steer Congress in the direction of what he considers responsible trade legislation has been buoyed by two months of upbeat news from the trade front that indicates a turnaround after four years of record deficits.

While the improvement in the current account figures was slight, it ended a string of quarterly increases in the deficit that carried throughout last year. John Hagens of Wharton Economics called the first-quarter figures "very good news" because "we are essentially standing still."

Nonetheless, the figures show the United States continued its position as the world's largest debtor nation, far surpassing the near $100 billion that debtor nations such as Brazil and Mexico owe.

The figures showed that foreigners continued to pour money into the U.S. stock and bond markets. Net foreign purchases of U.S. securities increased by $6.2 billion, to $18.5 billion from $12.3 billion, the Commerce Department reported. Record purchases of U.S. stocks were bolstered by rising prices even though the cost in foreign currency was lowered because of the fall in the value of the dollar.

While stock and bond purchases from overseas grew, foreign direct investment in the United States decreased from $12.6 billion to $9.3 billion, the Commerece Department said. Government economists said the decline was due in part to a spurt of investment in the fourth quarter of 1986 to take advantage of the old tax law.

All told, private foreign assets in the United States dropped by $44 billion to $13.4 billion in the first quarter of this year, from $57.4 billion in the last three months of 1986. Total foreign assets in the United States, which includes holdings by other governments, decreased by $30.8 billion. Foreign governments' assets in this country increased by $14 billion.

As a sign of the high foreign investment in the United States, income to overseas owners increased by $2.7 billion.

The Commerce Department also noted a $1 billion dip in transfers overseas, which it said reflected cuts in U.S. foreign aid to less developed countries.

The report also said that the United States spent $2 billion in January, February and March to support the dollar, about half of what has been spent on currency intervention so far this year. But the increase in foreign government assets in this country was credited on massive purchases of dollars by foreign central banks to keep its value from falling too sharply.