By 94 to 3, the Senate yesterday approved a three-year extension of the Endangered Species Act, but with one important, possibly weakening, change.

The change calls for creation of a seven-member Cabinet-level committee empowered to arbitrate conflicts between endangered species and federal public works projects and, in cases of unbreakable impasses, to exempt the projects from the law.

Sponsors of the committee idea, Sens. John Culver (D-Iowa) and Howard H. Baker Jr. (R-Tenn.), argued that their amendment would resolve growing and emotional objections to the law, due to expire Sept. 30.

Although the act has stopped completion of only one federal project, fear that other projects might be scuttled in wholesale lots moved the Senate to action.

Throughout three days of debate, it heard repeated warnings that unless changes were made, unspecified conflicts would throw a hammerlock on innumerable projects, increase unemployment and stop "progress."

The warnings gained currency with the Supreme Court decision last month holding that the Tennessee Valley Authority was illegally building its Tellico Dam because it was failing to protect the endangered snail darter fish.

Yesterday's vote was a sharp setback to the Carter administration, environmentalists who oppose any changes, and Sen. Gaylord Nelson (D-Wis.), who led a losing battle to keep the law intact.

Adding salt to their wounds, the Senate crushed a proposal by Nelson yesterday to limit the review committee's scope to projects on which there had been an "irretrievable" expenditure of money. The vote was 70 to 25.

Nelson's environmental allies, although reconciled to the Culver-Baker approach, were not pleased with its adoption. They fear that conflicts now being resolved under the law would be moved into a new political arena with the exemption power that would be granted to the new committee.

Michale Bean of the Environmental Defense Fund, part of a coalition attempting to extend the law without change, said that without the Nelson safeguards, there is great potential for undermining the act.

"What that committee cannot do is distinguish bad-faith consultations from hard consultations," Bean said. "Agencies will affected projects will think they have a better chance by not bargaining in good faith and getting their case before the committee. Culver's intentions are genuine, but we don't think it will function as he feels it will."

Bean and others said their lobbying efforts now will be concentrated on the House, where floor action on extension of the act is expected within the next month or so.

Before final passage, the Senate adopted more than two dozen technical and refining amendments to the bill. The key issue yesterday was Nelson's proposal to limit the review committee's activities.

Nelson noted that the existing consulation process has been "remarkably successful," resulting in accommodation on scores of projects that saved a plant or animal from extinction yet allowed completion of a dam or road.

But, he said, the Culver-Baker amendment would apply to all projects - not just those like Tellico, which were well on their way to completion - and that limits should be applied.

"Culver-Baker is overly broad. It threatens to destroy the consultation process that has workes so well," Nelson said .

According to the Interior Department, which administers the law, 677 plants and animals are listed as endangered or threatened. Of those, 235 are in the United States. Critics of the law contend that hundreds more plants and animals are expected to be added to the list soon, raising the potential for more conflicts between species and projects.