The federal government will pick up a share of the more than $1 billion cleanup tab at the crippled Three Mile Island nuclear power plant, Office of Management and Budget Director David A. Stockman has announced.

Stockman said the government will spend "in excess of $100 million" over the next several years to decontaminate the plant near Harrisburg, Pa., which was the site of the nation's worst commercial nuclear accident on March 28, 1979.

"This is a messy, unfortunate, urgent situation that needs to be addressed," Stockman said. "We are looking for a pragmatic solution." He said the government's contribution would be used to cover the cost of removing the contaminated reactor core and disposing of the radioactive materials.

The Reagan administration has had a recommendation under review since February to share the cost of the cleanup operation. It was proposed by the Nuclear Safety Oversight Committee, a panel established by President Carter to deal with financing the cleanup.

Until now, Stockman said, the administration had been reluctant to commit itself because there had been no guarantee that other parties would also contribute.

In recent weeks, the administration has been working closely with Pennsylvania Gov. Richard Thornburgh, who this summer proposed a cost-sharing plan calling for the federal government to pay $190 million over the next six years; the nuclear industry to spend $190 million; the plant's owner, General Public Utilities Corp., $245 million, and the states of Pennsylvania and New Jersey, a total of $45 million.

The balance of the cleanup cost would be paid by insurance. Regulators so far have rejected all proposals to pass the cost on to consumers.

Expected to take until 1986, the cleanup has been estimated as high as $1.3 billion.

Stockman confirmed the government's decision to accept the "framework" of the Thornburgh proposal at a press conference Friday night following a speech to a Republican group in Annville, Pa., about 15 miles from Harrisburg. He said, however, that he was not yet prepared to commit himself to any specific figures.

Earlier Friday, President Reagan told Thornburgh during a telephone conversation that he "agreed that the cost-sharing approach makes sense," according to a spokesman for the governor.

"It would be a mistake to say at this point that the president has agreed to assume the $190 milllion, but the governor was very encouraged by the conversation," the spokesman said.

The decision to help the nuclear industry clean up its worst accident was announced just a day after the administration unveiled a broad nuclear policy designed to revive the troubled industry.

Among the measures announced by the White House Thursday were a lifting of the ban Carter imposed in 1977 on the reprocessing of spent utility fuel; an easing of regulations to permit new plants to be planned and built in six to eight years rather than the present 10 to 14 years, and the commissioning of a study of the feasibility of obtaining plutonium through competitve procurement instead of relying solely on government-owned facilities.

"Nuclear power has become entagled in a morass of regulations that do not enhance safety but that do cause extensive licensing delays and economic uncertainty," the president said in a statement.