You mean this is the 3rd idler pulley or the third pulley they've replaced? If this is just the third pulley they've replaced, the Idler pulley can be a culprit in squealing problems. If the bearings go bad, then the belt will slip because the pulley isn't spinning smoothly and the shot bearings make it "catch".
If it's the third idler pulley, either your belt is eating them for lunch or your mechanic doesn't know how to put them on
