In a statement released by the US military on Saturday, the army confirmed two of its marines had died in Najaf while fighting militiamen.
A third was killed after his unit was ambushed in west Baghdad by resistance fighters opposed to the presence of foreign military forces in Iraq.
The soldier died when a rocket-propelled grenade slammed into his patrol vehicle on Friday afternoon, the military said in a statement.
More than 926 US troops have died in Iraq since last year's US-led invasion.