Oct 5 (Reuters) - Canada’s energy-heavy main stock index opened higher on Tuesday, as oil prices hit their highest levels in at least three years, with miners limiting gains. At 9:31 a.m. ET (13:31 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was up 40.87 points, or 0.2%, at 20,093.12. (Reporting by Amal S in Bengaluru; Editing by Vinay Dwivedi)