President Reagan is expected to move today against three countries and the European Community for alleged unfair trade practices, in an attempt to blunt bipartisan congressional pressure for protectionist legislation.

Administration sources said the president will accelerate investigations by U.S. Trade Representative Clayton Yeutter against Japan, Brazil and South Korea for blocking access for specific American products and services.

In addition, he will demand that Japan and the European Community end by Dec. 1 two types of trade practices that violate international trade laws.

Initial reaction on Capitol Hill was lukewarm as details of the presidential decision began to leak yesterday. "Democrats will continue to press for tough trade legislation," Rep. Tony Coelho (D-Calif.) said.

An aide to a Republican senator said at least some of the presidential actions are in areas with "limited export potential."

This is, however, Reagan's most forceful response to the public outcry over a trade deficit that is expected to reach $150 billion this year -- more than twice as high as it was two years ago.

It is, moreover, the first use of presidential authority to initiate trade actions under what Commerce Secretary Malcolm Baldrige called "such a strong, powerful law" that it has to be used rarely and with great care.

Under that law, Section 301 of the Trade Act, "the president has the authority to take almost any action you can conceive of in trade," Baldrige told reporters Thursday. Against opposition from other administration officials, Baldridge has urged the president for months to take these actions.

The president, though, did not use the full force of the law, which permits him to issue a finding of unfair trade practices without an investigation and to order immediate retaliatory actions.

"It sends an awfully clear message. The countries know they have got to do something or run the risk of retaliation. It's a warning. If they don't do something, he will act," one administration official said.

The president's announcement is scheduled two days after Senate Majority Leader Robert J. Dole (R-Kan.) warned that the trade deficit and the loss of U.S. jobs associated with it have become a potent political issue that threatens continued Republican control of the Senate.

The mood for action on trade intensified when the president last week overruled a recommendation by the International Trade Commission to protect the domestic shoe industry by blocking footwear imports.

Reagan is expected to unveil his trade moves during his regular Saturday noon radio broadcast, sources said. A mixup between the White House's National Security Council and the State Department forced the postponement of an announcement scheduled for yesterday morning.

According to administration sources, the State Department complained that it needed more time to notify the countries and the European Community facing the presidential action.

Presidential spokesman Larry Speakes already had announced that Reagan would come to the White House press room "to make a statement on international trade" followed by a briefing by Yeutter.

The president showed up, and on live television acknowledged his trade actions "are still under discussion."

Although the mixup was procedural, it was greeted with amusement on the Hill, where Democrats and Republicans commented that it was indicative of the disarray in this administration's trade policies.

The president selected five unfair trade cases out of a dozen given him Thursday by Yeutter after meetings this week of the administration's cabinet-level Economic Policy Council on the trade issue. There was disagreement within the administration, however, on which cases the president should move.

Sources said the president decided to order accelerated investigations to see whether international trade laws are violated by Brazil's restrictions on sales of foreign computers and communications equipment, Japan's tariffs on tobacco imports and South Korea's barriers to insurance sales.

The other two moves are expected to be against Japanese barriers to leather products and European subsidies on canned fruit and processed raisins, which have already been found to have violated international trade laws, sources said.

While none of the five items by themselves will have a great impact on the trade deficit, they range across varied sectors of the U.S. economy: high technology, agriculture and services.