By Gertrude Chavez-Dreyfuss and Joice Alves NEW YORK/LONDON (Reuters) – The dollar gained on Monday, rising for three straight sessions, as civilian killings in north Ukraine and the prospect of increased sanctions pushed investors to seek safety in the greenback. The U.S. currency also continued to benefit from a strong non-farm payrolls report for March that backed expectations for a hefty half a percentage point tightening by the Federal Reserve at next month’s meeting. “The dollar is bouncing higher as geopolitical developments have darkened clouds over the global economy,” said Joe Manimb…