* Senior oldest winner of all three of Australia's top events * Former world number one Scott finishes fifth * U.S. Amateur champion DeChambeau joint runner-up (Adds details) By Ian Ransom MELBOURNE, Nov 22 (Reuters) - Peter Senior outclassed a host of challengers half his age to clinch his third Australian Masters title by two strokes on Sunday, and at 56 became the oldest winner in the tournament's 36-year history. The stocky veteran overcame a bout of nerves and withstood a late charge by Sydney professional Andrew Evans at Melbourne's Huntingdale Golf Club before sealing the win on the 18th hole with a clutch putt for par. Senior's final round three-under 68 gave him an eight-under total of 276, sealing his third 'gold jacket' 20 years after his second at Huntingdale in 1995. More...