Aug 19 (Reuters) - Canada’s benchmark stocks index hit a three-week low on Thursday, with oil stocks leading the decline as crude prices sank on the back of a surge in COVID-19 cases. At 14:31 a.m. ET (13:31 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 103.9 points, or 0.51%, at 20,198.21. (Reporting by Sruthi Shankar in Bengaluru; Editing by Subhranshu Sahu) Our Standards: The Thomson Reuters Trust Principles.