The US military on Saturday said the three marines died in action in the Anbar province, which extends from just west of Baghdad to the Syrian and Jordan borders.
Also during the day, Centcom confirmed the deaths of two more soldiers, including one from the 81st Brigade Combat Team who was killed on Tuesday in a mortar attack south of the Iraqi capital, and a non-combat death in northern Iraq on Friday.
The mortar attack also wounded nine soldiers, the US military said.
As the occupation toll mounted, the US-appointed governor of Najaf accused Shia cleric Muqtada al-Sadr of failing to honour a deal to end fighting in the city.
Militias loyal to al-Sadr said they killed one Iraqi policeman and captured another on Saturday in a gun battle in the town of Kufa.
Governor Adnan al-Zurufi said al-Sadr had done little to stop his fighters from brandishing their weapons in public or to send home militiamen not from Najaf – key elements in the truce agreed to end fighting in the city.
"Unfortunately, there have been no positive initiatives from the office of Mr Muqtada al-Sadr so far," the government complained.
Sadr's fighters wielding machine guns and rocket-propelled grenades exchanged fire with US soldiers approaching the centre of Kufa.