The Teamsters union announced yesterday that its membership had narrowly ratified a new three-year contract with the trucking industry.

Later in the day, a federal judge rejected a dissident group's attempt to block the agreement from taking effect. The group had argued that the union should have counted the votes of part-time Teamsters, whose wages would be cut by the contract.

The union said the agreement, retroactive to March 31, would raise base wages of $13.21 an hour by 50 cents an hour this year and $1 an hour by the third year. It was approved by a vote of 62,296 to 54,873 -- a 53.2 percent majority.

After the announcement, three part-time truckers asked U.S. District Court Judge Charles R. Richey to block the contract, saying that the union had not counted the votes of its part-time members. Under the agreement, part-timers' pay will be cut from at least $13.21 an hour to $11.

Richey refused to rule on the request, but ordered the union to survey its 300 local units to see how many part-time truckers, known as "casuals," are Teamsters. Management will also make a survey of the number of casuals.

Richey ordered the union to report its findings to the court next week and scheduled a hearing for May 28.

The three Teamsters who filed to block the agreement, George Harmon of Pasadena, Md.; Albert Bornstine of Norton, Ohio, and Phillip Obershaw of San Bruno, Calif., are members of Teamsters for a Democratic Union, a dissident group that wants to oust the union's leadership. The group was represented by the Public Citizen Litigation Group, a legal services organization founded by Ralph Nader.

The group estimates that there are at least 35,000 part-time truckers, enough to alter the outcome of the vote.

Lawyers for both the Teamsters and the trucking industry contended in court that there are fewer than 7,000 part-timers and thus they wouldn't affect the outcome. The union said it disqualified 2,106 ballots.

Union lawyer Joseph Santucci also argued that the vote had already made the contract binding. Richey rejected that argument, however, saying that the agreement "will be implemented at your own risk."

The agreement, involving the Teamsters, Trucking Management Inc. and the Motor Carrier Labor Council, covers the nation's largest unionized trucking companies, as well as many smaller ones. The union said the agreement would affect about 200,000 truckers.

Under the contract, part-time workers and new employes would receive lower wages than senior members for the first time since a national trucking agreement was first negotiated in 1964.

Teamsters spokeman Duke Zeller told the Associated Press that part-time workers traditionally have not been allowed to vote on union contracts. But a lawyer for the dissidents, Arthur L. Fox III, said they should have been counted in this election.

In a statement, Teamsters President Jackie Presser said he was "pleased that Teamster members affected by the 1985 agreement have solidly approved the new settlement."

A representative of Teamsters for a Democratic Union in Detroit said the margin by which the agreement was approved "was narrow and represented a setback for them."

The spokesman, who asked that he not be identified "so I can get a job," said the 46.8 percent vote against the contract "is much larger than it's ever been before."