Dec 23 (Reuters) - Canada"s main stock index extended gains for the third consecutive session on Thursday, aided by energy shares, while early data suggesting the Omicron variant of the coronavirus was less severe than feared calmed investor nerves. At 9:30 a.m. ET (14:30 GMT), the Toronto Stock Exchange"s S&P/TSX composite index (.GSPTSE) was up 42.57 points, or 0.2%, at 21,112.62.