Three West Germans who were captured by Nicaraguan rebels last month appeared at a news conference here yesterday to recount their experience and call upon the U.S. government to halt aid to the rebels, known as contras.

"I appeal to the people of the United States to stop financing of contra terrorists," Reingard Zimmer, a 20-year-old German woman, said at the Capitol.

Congress voted $27 million in nonlethal aid to the contras last year. President Reagan proposed $30 million in nonlethal and $70 million in military aid this year for them; the House is expected to vote on the aid issue today.

Twelve West Germans were captured by the contras on May 17 in the small community of Jacinto Baca, 156 miles southeast of Managua, where they said they were living in an agricultural cooperative and constructing houses. Four of them escaped by crawling into the bushes, according to Sean Steinbach, 24, a medical student.

"We were held under psychological oppression thoughout our captivity," Zimmer said, describing an ordeal that alternately included punishment and rewards, hope and despair.

Dominik Diehl, 23, a medical student, said that one of the West Germans was suffering from hepatitis and was too weak to keep walking. But he said that a contra put a gun to the sick man's head and forced him to continue.

Diehl said the contras had U.S. boots and uniforms, and used U.S. Army communication equipment. "I assume that is what they call the U.S. humanitarian aid," he added. The contras also carried West German and Belgian arms, he said.

The Germans denied contentions by the State Department that they had been armed and living on a military base. "We went to Jacinto Baca to build houses to support the local farming cooperative, but the contras came to destroy," Steinbach said.

The news conference was called by Oxfam America, an international relief and development agency, and was cosponsored by Witness for Peace, the Washington Office on Latin America, and the Coalition for a New Foreign and Military Policy.

John C. Hammock, executive director of Oxfam America, said the "so-called humanitarian aid" to the contras jeopardizes the development projects in Nicaragua because of contra attacks. Sam R. Rope, spokesman for Witness for Peace, said, "Epidemics are breaking out in remote areas and 159 children died from measles in three small communities in two months this year."

The three Germans said that they would return to Managua and rejoin their friends. Three others in the captured party have left Managua for West Germany.