Three key Democrats today will unveil legislation that could result in the imposition of a stiff import surcharge against goods from nations that run large trade surpluses with the United States, congressional aides said.

The legislation, sponsored by House Ways and Means Chairman Dan Rostenkowski (D-Ill.), Sen. Lloyd Bentsen (D-Tex.) and Rep. Richard Gephardt (D-Mo.), would place a 25 percent surcharge on imports if a series of intermediate steps fail, Hill aides said.

It also is intended as a warning to the Reagan administration to get tough on trade, they said.

The bill is the latest addition to a mound of protectionist legislation on Capitol Hill that is aimed at countries with large trade surpluses with the United States, such as Japan.

Many members of Congress have said they are disappointed with what they consider the disjointed and weak efforts of the administration to force other countries, particularly Japan, to open their markets to American goods. The Japanese in the past few weeks have introduced measures aimed at defusing protectionist pressures in the United States by reducing tariff and nontariff barriers. However, U.S. trade officials have said the measures so far are either too vague or are not substantial.

The Rostenkowski-Bentsen-Gephardt bill stresses giving the office of the U.S. Trade Representative a central voice in trade policy to make the administration more aggressive in pursuing policies more favorable to the United States, congressional aides said.

The bill is intended to affect countries whose exports to the United States exceed their imports from this country by 65 percent. Those countries' total world exports also would have to be 50 percent greater than their total imports.

The surcharge would be removed only if a country's trade surpluses were reduced an additional 10 percent every year after the surcharge became effective.