The chairmen of the Big Three auto companies lent their support to President Carter's proposed energy plan today, while warning of the difficulties it will impose on consumes.

At an early morning news conference General Motors chairman Thomas A. Murphy voiced satisfaction with the adoption of a broad conservation policy but criticized the tax and rebate provisions for penalizing families who need bigger cars.

Murphy repeated his comment made Wednesday that a gass guzzler tax is a "simplistic approach," but toned down his earlier criticisms. "To me, the President has a positive program," he said. "He intends to . . . develop a comprehensive energy policy that will stand this nation in good stead.

Henry Ford II, who has repeatedly announced his support of higher gasoline taxes during the past three years, said while "the President has offered the nation some tough remedies for energy problems . . . we all must take part in the tast of resolving it.

"The burdens should be spread fairly and equitably so that no single sector of the economy is penalized," he added.

The ford Motor Co. chairman said, "It is clear he has singled out this industry for attention," but added that the auto makers are already fully committed to meeting the fuel economy mileage standards.

Americans Motors Corp. chairman Roy D. Chapin Jr., applauded the Carter action while characterizing it as "a tough challenge to the American people.

"What the President is proposing parallels what American Motors has been saying for a long time," Chapin said.

Spokesman for two major foreign car makers, Toyota and Volkswagen, expressed similarly positive, but cautious reactions.

A VW spokesman said the company's cars currently meet the 1985 fuel economy requirements.

But, he added, "we are concerned that the President has separated imported cars from domestics."

A statement by Toyota operations vice president Norman D. Lean said the President's proposals "may help the longer term outlook for small, fuel efficient cars." Lean added, "Of course we are hopeful that Congress would not discriminate against the imports in any tax rebate program." He noted that 130,000 Americans are employed by foreign car importers in this country.

The President's program is reportedly incomplete as it affects imported cars, apparently because some major international trade issues remain unresolved.

Representatives of both domestic and foreign car companies said yesterday they understood that no rebates would be given for imported cars unless offsetting taxes are levied against other cars from the same country.