By Florence Tan SINGAPORE (Reuters) -Oil prices extended gains for a third session on Monday, with Brent rising above $81 a barrel to its highest in more than four months, as wider U.S. sanctions are expected to affect Russian crude exports to top buyers China and India. Brent crude futures climbed $1.48, or 1.86%, to $81.24 a barrel by 0113 GMT after hitting an intraday high of $81.49, the