The Justice Department will investigate whether federal law was violated when unauthorized changes were made in the transcripts of a 1982 congressional hearing on the Environmental Protection Agency, three Republican House members said yesterday.

Rep. Robert S. Walker (R-Pa.) called Justice's decision "a very dramatic step" in the case, bearing out his repeated contention that the transcript changes were serious offenses, not mere congressional curiosities.

Alterations in the transcripts of July, 1982, hearings in ways that made Walker and other Republican members look foolish are already under investigation by the House Committee on Standards of Official Conduct and have led to the resignation of Lester Brown, a Government Operations subcommittee staff member.

Government Operations Chairman Jack Brooks (D-Tex.) said Brown admitted making unauthorized transcript changes, though not necessarily all those or the particular ones at issue in the dispute.

Walker and other Republicans asked the Justice Department to enter the case before Congress went on vacation in August.

Brown, in a statement issued through his attorney, said the fact that the Justice Department was looking into the matter is known and said that the "ballyhoo" being given the investigation by Walker and the other Republicans "is evidence that an attempt is being made to bring political interference to bear on an investigation that would otherwise be handled administratively."