Oct 1 (Reuters) - General Motors Co on Friday reported a drop in U.S. auto sales for the first time in four quarters, hit by worsening semiconductor chip shortages and low inventories. The No. 1 U.S. automaker said here third-quarter sales fell to 446,997 vehicles, down 218,195 vehicles from a year earlier. (Reporting by Abhijith Ganapavaram in Bengaluru; Editing by Maju Samuel) Our Standards: The Thomson Reuters Trust Principles.