By Dominic Lau TOKYO, Aug 17 (Reuters) - Japan's Nikkei share average rose to a three-month high on Friday as investors' risk appetite grew after German Chancellor Angela Merkel appeared to back the European Central Bank's efforts to tackle the euro zone debt crisis. Expectations of further impending action from the ECB to fight the region's debt crisis and recent stronger-than-expected U.S. economic data have helped the Nikkei to rally from a seven-week trough hit on July 25.