By Martyn Herman RIO DE JANEIRO, Aug 18 (Reuters) - Australia's Sam Willoughby stayed firmly on course for BMX gold as three of his main Olympic rivals were eliminated after a spate of crashes in the quarter-finals on Thursday. Willoughby won all three runs of his heat, avoiding the kinds of problems that accounted for Latvia's double Olympic champion Maris Strombergs, world champion Joris Daudet of France and former world title winner Liam Phillips of Britain. More...