BAGHDAD — A roadside bomb struck a U.S. convoy in western Iraq, killing three Americans, including a senior State Department official, U.S. officials said Tuesday. The blast killed Terence Barnich, the deputy director of the State Department's Iraq Transition Assistance Office in Baghdad, as well as a U.S. soldier and a civilian contractor as their convoy left a construction site near Fallujah on Monday, military and government officials said.