In a statement on Monday, occupation forces said all three soldiers were from the 1st Armoured Division. The incident took place around 1440 GMT on Sunday.
About 811 US soldiers have died since the start of the US-led invasion of Iraq in March of last year, according to a tally based on Pentagon figures.
Earlier, a US patrol came under attack in Najaf on the fourth day of a fragile ceasefire with Shia cleric Muqtada al-Sadr and his militia.
The clashes, which started around 0900 GMT and lasted about one hour, were centred around the northern end of the city's vast cemetery - known as the Valley of Peace.
In Baghdad, at least two Iraqis were killed and five wounded when unknown assailants fired at a convoy of three 4WDs on a busy road in northwestern Baghdad, a police officer said.
Meanwhile, north of the capital, two men were killed when a home-made bomb they were planting by a road near the oil refinery centre of Baiji exploded prematurely.
And Britain's Foreign Ministry said four of its nationals escaped unhurt, but one Iraqi was killed, after unknown assailants ambushed a convoy on a northwest Baghdad highway.
US officials have repeatedly said they fear an upsurge in attacks before the 30 June partial handover of sovereignty to a selected government.