March 30 (Reuters) - Canada’s main stock index opened lower on Tuesday, dragged down by mining stocks that slipped nearly 2% as bullion prices fell to a three-week low, while energy stocks tracked weaker crude prices. * At 9:31 a.m. ET (13:31 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 66.48 points, or 0.36%, at 18,652.74. (Reporting by Amal S in Bengaluru; Editing by Ramakrishnan M.)