Three environmental leaders yesterday accused Interior Secretary James G. Watt of "doublespeak," contending that he spoke of balanced use of natural resources while acting as a "front man for exploiters" who could undo two decades of bipartisan support for conservation.

In a meeting with reporters and editors of The Washington Post, leaders of the Wilderness Society and the National Audubon Society contended that Watt had misinterpreted President Reagan's electoral victory, misreading campaign sloganeering about over-regulation as a mandate to step back from government efforts to protect the environment.

Former senator Gaylord Nelson, a Wisconsin Democrat who was ousted in the Republican landslide and now heads the Wilderness Society, predicted that Watt will begin running into heavy political opposition in his attempt "to turn back the clock" at the Interior Department.

But Nelson, who described Watt as an ideologue "unfit to hold public office," also said the interior secretary could override public and congressional opposition and make revolutionary changes in land, water and resource policy through administrative fiat.

Environmental groups have responded to Watt's early actions at Interior with near horror and unusually harsh language.

Yesterday, William Turnage, executive director of the Wilderness Society, called Watt a "malign" influence in a Cabinet post that has an almost schizophrenic job definition as protector and user of national resources.

Dr. Russell Petersen, president of the National Audubon Society and a former Republican governor of Delaware, said Watt is using an almost "diabolical approach" in describing himself as the "good steward" of the nation's resources while, as Petersen put it, "representing the exploiters."

Petersen said Reagan's appointment of Watt, who directed the anti-environmentalist Mountain States Legal Fund before coming to the administration, "brought into government a force the environmentalists have been fighting" for more than a decade.

"It is like putting socialist in charge of the Department of Commerce," Petersen said.

The three environmental leaders accused Watt of playing loosely with facts in some of his early moves to alter Interior policy, especially in the effort to place a moratorium on land acquisitions for national parks.

Watt has argued before Congress that money from the Land and Water Conversation Fund, set up to use offshore-oil royalties to buy park land, should be diverted temporarily to park maintenance.

Turnage said Watt's argument that national parks are run down and unsafe was based on misleading studies about water supplies, roads and lodging facilities in the parks.

He contends that the major problem in the parks is overcrowding, a situation that made a better case for adding new park lands as directed by the law creating the Land and Water Conservation Fund.