WASHINGTON (AP) _ Three New England senators unveiled legislation to combat air pollution on Monday and said the Reagan administration is on the verge of a major policy shift on acid rain policy.

Sens. Robert Stafford, R-Vt., John Chafee, R-R.I., and George Mitchell, D- Maine, said the change would come this week with what they said is President Reagan's expected approval of a U.S.-Canadian proposal to speed development of clean coal-burning technologies.

They told a news conference that a decision to spend $5 billion on ways to reduce chemical emissions from industrial boilers would be a reversal of the administration's insistence that more scientific study is needed to determine whether acid rain is harming the environment.

''It's an admission that the problem is a real one,'' said Chafee, chairman of the Senate Environment subcommittee that handles acid rain legislation.

''We've turned a corner ... on the subject of controlling acid rain,'' Stafford said. ''It is no longer a question of whether, but when and how.''

Stafford, chairman of the Environment Committee, said he would be quite surprised if the clean-coal proposal is not accepted by Reagan this week when the president meets with Canadian Prime Minister Brian Mulroney.

Mitchell, one of the more active Democrats on the Environment Committee, said U.S. acceptance of the proposal would ''represent a total reversal of the president's position and a major step forward. It should terminate the debate on whether there is an acid rain problem.''

Meanwhile, the International Maple Syrup Institute, the American Maple Syrup Council and the Sport Fishing Institute released letters to Reagan at a newss conference jointly sponsored by the Clean Air Coalition, asking the president to support measures to reduce the precursors of acid rain.

David Marvin, president of the institute, said he was convinced by circumstanstial evidence that acid rain was killing maple trees in northern New England and eastern Canada. Although a report from the National Research Council last Friday said there was no evidence that acid rain was harming forests, Marvin said, ''If we treated human health the way we treat trees, we'd still be supporting smoking.''

On the other side of the debate, the Hudson Institute, an Indianapolis, Ind., think tank founded by the late futurist Herman Kahn, held a news conference on its own survey of the acid rain debate that concluded ''there is no clear and unambiguous cause-and-effect relationship that makes (acid rain) undeniably guilty of any of the major accusations which have been leveled against it.''

The National Research Council report had concluded there was indeed a clear cause-and-effect relationship between acid rain and the acidification of some lakes, notably in the Adirondack Mountains of New York state. William M. Brown, principal author of the Hudson Institute report, said he had not seen the council's work.

The Regan-Mulroney talks will involve a recommendation for development of clean-coal technology that came out of a year's study by envoys from the two nations. Canada, arguing that its lakes and forests are being damaged by emissions from U.S. factories and utilities, has been pushing for action in this country.

Mitchell said he believes Reagan will endorse the joint plan if for no other reason than it is ''absolutely essential'' to the domestic political fortunes of Mulroney, who has pursued a non-confrontational approach with Reagan on the acid rain question.

A shift on the part of the administration could help isolate congressional foes of acid-rain control legislation, who have successfully bottled up proposals with the same argument - that more study is needed.

Stafford said, however, he thinks the Senate, at least, has reached a ''turning point'' because ''a considerable number of senators'' are now taking the problem ''very seriously.''

Chafee observed that while some scientists still have doubts about acid rain's role in environmental damage, ''there's still a group challenging whether Shakespeare wrote all those plays.''

The legislation unveiled Monday not only would attempt to reduce industrial emissions of the main ingredients of acid rain - sulphur dioxide and nitrogen oxides - but also cut hydrocarbon and carbon monoxide emissions from vehicles.

It would require all autos within five years to have exhausts as clean as those of the cleanest 25 percent of the 1986 models and for automakers to double the current five-year, 50,000-mile factory warranty on emissions control equipment.

The bill also would mandate that the United States negotiate air pollution control treaties with Canada and Mexico. Some congressmen have argued that emissions from huge copper smelters in Mexico are damaging the Rockies.