The Chicago Tribune, the nation's fourth largest newspaper, published slightly thinner editions today with little apparent trouble despite a strike by three production unions.

The walkout, which began Thursday night and involves about 1,000 typographers, mailers and pressmen, centers on a labor-management dispute over the company's attempt to reassign senior union typesetters from obsolete jobs to new duties.

Both sides met in bargaining sessions today, and a representative of the Tribune said it will continue publishing despite the dispute. "We're still going to come out. We're still printing. We will have a pretty good Sunday edition," she said.

Under a 1975 agreement, the Tribune guaranteed lifetime jobs for its printers in return for bringng in new techonology. The unions insist that this agreement does not now give the company the right to reassign employes.

Robert A. Handley, president of the Chicago Web Printing Pressmen's Union Local 7, which represents some 350 workers, said, "They want us to agree to gut our contract. I'm hurt that a company would treat me like that, just throw you to the winds and say they don't need you after 45 years on the job."

Tribune management in a series of public statements said it was surprised by the walkout. In an article today, the paper quoted its president, Charles T. Brumback, as saying, "We were hopeful the issues could have been settled without a work stoppage."

The key to the strike may lie with the Teamsters, who have refused to honor the picket lines outside the Tribune's modern Freedom Center printing plant west of the Loop. Teamster Local 700 leadership passed out leaflets saying their contract prohibits them from joining the strike.

Teamster-driven trucks crossed the picket line today, despite ang- ry shouts of "Scab!" from the strik- ers.

In addition to losing pages, the Tribune was unable to publish in full color today. Color photos and graphics "are a very time-consuming, costly process," the representative said. "Right now, we are putting our resources elsewhere."