A congressional campaign to eliminate baseball's antitrust immunity and to restrict the use of territorial veto rights in all professional sports gained momentum yesterday.

Representatives of three players unions joined three congressmen and Sen. Alan Cranston (D-Calif.) at a press conference to promote the Sports Antitrust Act, aimed at ending what they called the monopolistic powers of leagues.

The bill, introduced in February by Rep. John F. Seiberling (D-Ohio), would end baseball's 57-year-old exemption from antitrust laws and make it illegal for a professional team in a large metropolitan area to bar other teams in that league from its area.

Specifically, the bill says that one team cannot bar another in the same league from locating within a 75-mile radius unless that area has a population under 2 million.

The 75-mile radius was used because most professional sports, except soccer and baseball, use that radius for defining home territory. Baseball and soccer uee a 100-mile radius.

A companion to the bill, which has about 20 sponsors in the House, was introduced in the Senate this week by Cranston.

Supporting the bill yesterday were Marvin Miller of the Major League Baseball Players Association, Brig Owens of the National Football League Players Association and John Kerr of the North American Soccer League Players Association. Representatives of the basketball and hockey players unions could not attend.

The press conference was called by Rep. Julian C. Dixon (D-Calif.), a co-sponsor whose Los Angeles constituents may see the Rams move to Anaheim after the next season with no prospects for a replacement because of the territorial veto rights.

Seiberling, an antitrust lawyer, said House Judiciary Committee Chairman Peter W. Rodino (D-N.J.) has promised to hold hearings on his bill.

"If it gets to the floor, it will pass overwhelmingly," Seiberling said.

A similar situation involving the Baltimore Orioles' control over American League territorial rights for Washington has helped keep this city without baseball since 1971 and prompted Rep. Herbert E. Harris (D-Va.) to cosponsor the bill.

"Proffesional sports enjoy the best of all possible worlds: public adulation without public accounting," said Dixon. "Since they are essentially self-regulating monopolies, sports franchises are able to come and go as they please, exacting numerous comments along the way.

"In all of this, the sports fan - and the taxpayer - are the ultimate losers."