Three protesters were arrested outside the Soviet Embassy yesterday when they refused to leave after trying to present a petition urging a halt to Soviet support of Vietnamese attacks in Cambodia, authorities said.

Shortly before the three men were arrested on charges that they violated a law prohibiting demonstrations within 500 feet of an embassy, an estimated 200 members of the Cambodian-American community gathered in Lafayette Square to denounce Vietnam's six-year-old invasion of their Southeast Asian nation. They also called on the United States for increased aid.

"We are concerned about what happens to our brothers who are suffering because of a Vietnamese slaughter," said Chhut Chhoeur, a former Cambodian ambassador to the United Nations who now speaks for the newly created Cambodian-American Foundation.

"History has shown that Vietnam and communists understand only force," he said. "We are appealing to you. Arms can be shipped. . . . You have helped Afghanistan and Ethiopia, so please help us."

Chhoeur emphasized that the Cambodians are not asking for direct U.S. military involvement.

A three-party resistance coalition, recognized by the United Nations as Cambodia's legal government, has been fighting the Vietnamese army.

Linas Kojelis, assistant director of ethnic affairs for the White House, told the protesters that President Reagan supports them, but did not make any promises.

"You are the forces of good," Kojelis told the demonstrators. "The President supports your struggle for freedom."

The arrests came after the demonstrators left the park and marched as close to the Soviet Embassy on 16th Street NW as the District's 500-feet rule allows. Four demonstrators broke away and made their way to the embassy.

They were met by a Soviet official who refused to accept their petition and asked them to leave.

The Soviet official said, "I will not accept your petition. We are not responsible for the doings you are alleging."

Minutes later, U.S. Secret Service agents arrested the three men, identified as Sanguwar Delopez, Dara Peon and Phanna Nhek.