* Froome extends lead to 2:27 over Mollema * Australian Porte on the rise, Quintana dropped * Russia's Zakarin bounces back to win stage 17 * Van Garderen, Valverde out of podium contention (Writes through with quotes) By Julien Pretot FINHAUT-EMOSSON, Switzerland, July 20 (Reuters) - Britain's Chris Froome took another big step towards a third Tour de France title when he dropped his main rivals in the finale of the 17th stage, a 184-km mountain trek won by Russian Ilnur Zakarin on Wednesday. Froome (Team Sky) attacked inside the final two km in the last ascent, a punishing 10.4-km climb at an average gradient of 8.4 percent and Dutchman Bauke Mollema, second overall, as well as last year's runner-up Nairo Quintana, could not follow. More...