Three former Reagan administration ambassadors called on President Reagan yesterday to replace Secretary of State George P. Shultz, charging that the State Department is "undermining President Reagan's foreign policy."

Speaking at a forum of the conservative Heritage Foundation think tank were David B. Funderburk, ambassador to Romania from 1981 to 1985, Charles M. Lichenstein, alternate ambassador to the United Nations from 1981 to 1984, and Curtin Winsor Jr., ambassador to Costa Rica from 1983 to 1985. Two officials of the foundation, Vice President Burton Yale Pines and publication editor James T. Hackett, also called for Shultz's ouster.

Funderburk, now a professor at Campbell University in North Carolina, said State Department behavior toward Romania is "tragically wrong . . . a pro-slavery policy" in contrast to Reagan's stated policy of "developing human rights and freedom from Soviet domination."

He charged that a network of State Department career officers "works frantically to denigrate information from the field" on Romanian violations of human rights. "Shultz only knows what the network tells him. I don't think he effectively represents Reagan's foreign policy," Funderburk said.

He charged that Romania provides haven for "thousands of radical Arabs and other terrorists" and aids guerrillas worldwide, and said its ambassador to Washington is "one of the great liars of the universe." A spokesman for the Romanian Embassy said the charges were "a very dirty calumny."

Winsor, now a Washington consultant, criticized Shultz's opening last year of direct talks with the leftist government of Nicaragua. "They could have had only one outcome: the sellout of Honduras and our other friends in the region," Winsor said. The State Department favors "internationalist thinking" at the expense of U.S. interests, and misleads U.S. allies, he said.

"If they cannot trust us to clear out the bear-infested garbage from our own front yard, how can they expect us to fulfill our other obligations?" he said.

Lichenstein, now a senior fellow at the foundation, said career diplomats "control the supply of both carrots and sticks" within the State Department and foil policies they dislike by outwaiting political appointees, who depart when administrations change. This is "tolerated" by Shultz, Lichenstein said. Asked why, he said: "We have the wrong secretary of state."

Lichenstein said perhaps Reagan "is not fully aware of the nature of the diplomatic process," which involves many "nuances" in field operations. He advised questioners to write Reagan and make their feelings known.

Hackett said any president elected to change things "must appoint supporters dedicated to that goal."

Pines said after the forum that the former ambassadors "are implicitly criticizing Ronald Reagan for keeping Shultz on." He said Shultz ought to be named head of the Federal Reserve Board. "The country would benefit both economically and in terms of foreign policy," he said.