The United States ran a $33.67 billion deficit in the broadest measure of its foreign trade and financial transactions for the first three months of the year, a slight improvement over the record deficit posted in the last quarter of 1985, the Commerce Department reported yesterday.

While the January-March measurement of the U.S. current account in international transactions showed a leveling, the deficit remained at a much higher level than it was a year ago. The deficit for the first quarter of 1985 was $26.1 billion.

The current account measures trade in merchandise and services as well as financial flows and investments between the United States and the rest of the world.

It showed that the United States remains a debtor nation -- a status it reached in 1985 for the first time since World War I -- with foreigners owning more U.S. investments than Americans own overseas.

Economists believe final figures for last year -- due to be released later this month -- will make the United States the world's third-largest debtor nation, behind Brazil and Mexico, which each owe about $100 billion.

Even with the leveling off, the current account deficit this year is expected to reach $135 billion -- higher than last year's record of $117.7 billion -- and the United States is likely to become the world's largest debtor nation sometime in 1986. The total debt could reach $400 billion before improving.

Nonetheless, C. Fred Bergsten, director of the Institute for International Economics, predicted that the deficit has peaked, with the figures from October until March reflecting a continuing flood of imports as well as the price increases caused by the fall in the value of the dollar.

"It's the worst of both worlds," he added, with trade flows "dominated" by the strong dollar of late 1984 and early 1985. This drew in cheap imports and made American products so expensive overseas that they couldn't compete.

"I don't think the dollar has gone down far enough" to end four years of record U.S. trade deficits, Bergsten said. "But we will now start seeing declining levels of external deficits."

Another economist, Michael Evans of Evans Economics, called the leveling off of the deficit "misleading" because of the sharp drop in oil prices over the past six months. "We should have done a lot better than remain stable in the last quarter," considering how low oil prices are.

The Commerce Department study showed that Americans were investing their money overseas in record amounts. U.S. purchases of foreign securities hit a record $6.1 billion in the first quarter of 1986, which is more than four times greater than in the fourth quarter of last year. Americans bought $2.1 billion in foreign stock, with more than half the purchases taking place in Japan.

Evans said that showed a deliberate policy on the part of Wall Street investment advisers, who have decided that foreign securities are a good deal for American investors. "Foreign securities are the place to be, so we will see big numbers in the future," said Evans.

At the same time, the figures showed foreigners continued buying American securities even with the weaker dollar. That cash inflow is needed to finance the U.S. budget deficit.

The first-quarter figures showed a slight, $800 million decrease in the merchandise trade deficit, which ran $36.6 billion. Exports increased $800 million, to $53.5 billion, while imports held steady at $90.1 billion. A decrease of $4.1 billion in oil imports was offset by a record $80.1 billion in non-oil imports.