Major orchestras don’t lose their principal musicians very often. After all, these are the plum jobs of the music industry, high-salaried and high-profile. But this year, the Cleveland Orchestra will lose no fewer than three of its principal players, and the scramble for their jobs is on. Two of the three principals are retiring, and the other, who had moved up from second chair, is returning to his old position after not being tenured at the principal position.