ALTHOUGH its French parent company is shedding workers, Groupe Bull's United States circuit-board plant is expanding into a third production shift, so that some of its assembly lines will operate around the clock.Bull HN Information Systems Inc. celebrated the reopening of its Brighton, Mass., factory July 19, following a $17 million retooling and reorganization. The plant makes circuit boards for Bull mainframe computers and minicomputers, and for personal computers made by Zenith Data Systems, which Groupe Bull purchased from Zenith Electronics Corporation last year. The circuit boards, on which memory and microprocessor chips are mounted, are shipped for use in machines assembled in North and South America, Europe, and Asia. Bull has one other circuit-board plant in Angers, France. The retooling here represents a consolidation of Bull's circuit-board manufacturing, involving a politically sensitive shift of some work from Bull's home turf in France, as well as from Scotland and Phoenix. Also, the Brighton plant now makes all the circuit boards for the Zenith Data Systems personal computers; an assembly line for the boards was moved from Zenith's computer plant in St. Joseph, Mich. Joel Beck, vice president of US manufacturing for Bull HN, says new machines and work methods have improved productivity at the plant dramatically - from an annual figure of $300,000 per worker to $800,000 per person.