A third of the members of the Public Employes Relations Board, which mediates labor disputes for the Prince George's County government, are quitting out of dissatisfaction with the labor relations practices of County Executive Lawrence J. Hogan, sources said yesterday.

Three of the independent board's nine members wrote Hogan this week to say they will leave the board July 31.

The three, Chairman Howard Gamser, Jacob Seidenberg and Laurence Seibel, have served on the board since it was established seven years ago and are nationally recognized experts in the field of arbitration.

They would say only that they have decided to leave because the board has taken up too much of their time. They said the catalyst for their decision was a disagreement between the county and the American Arbitration Association, which administers the board and finds members for it, over a new contract. The current contract expires July 31.

Sources said the more basic cause of their resignations was their feeling that Hogan has made labor relations too political and created an "inhospitable atmosphere" for settling labor disputes with the county government.

In addition, many board members feel that Hogan distrusts the labor board and would like to replace it because of a ruling last March by one board member that Hogan was guilty of unfair labor practices in his handling of a dispute with the county's 1,500-member public employes union. That ruling was reaffirmed last week by a Prince George's County Circuit Court judge.

One board member, who refused to be identified, said he believes Hogan would like to get rid of the board because "We're kind of a bone in his throat. We're independent. Now he can appoint his allies who he can control."

Another board member, also requesting anonymity, said, "Mr. Hogan believes it's politically advantageous to beat unions and the [labor board] is standing in his way. In all my experience I've never seen this kind of purposeful political action."

County Attorney Robert Ostrom confirmed yesterday that he had been unable to reach an agreement on a new contract with the arbitration association. He said the labor board has been too expensive and the county is looking at alternatives to the board.

He disputed the claim that the county was intentionally trying to replace the board because of Hogan's dissatisfaction with its rulings.

"Of course I'm upset about the criticism," he said. "I don't think it's justified. I think they're overacting. They're too sensitive."

Ostrom said, "We're in an analysis period about what a netural third party group should be about. But the message is clear that the county still wishes (to provide such a group) to handle labor matters." Hogan who is attending the Republican National Convention in Detroit, was unavailable for comment.

In addition to the 1,500-member public employe union, largest in the county, Hogan is involved in a contract dispute with the county police union.