The bodies were discovered on Wednesday by a group of Iraqi National Guardsmen shortly after dawn as they patrolled near the town of Dujail, 60 km north of the capital.
The US military said initial indications were that the dead men were Arabs.
The development comes after a sharp surge in violence in the past week, with at least 170 Iraqis killed in bomb blasts, clashes and other attacks. A car bomb in central Baghdad on Tuesday killed 47, the worst single death toll in six months.
Iraqi police said two of the bodies had tattoos written in the Roman alphabet - one saying "HECER", and the other a letter H. The third body had tattoos written in Arabic script but the words were not Arabic. There were no documents on the corpses.
Clothed in T-shirts
Two were clothed in jeans and T-shirts and the third was wearing sweatpants and a T-shirt, the US military said.
Dozens of foreigners have been seized in Iraq in recent months, many of them truck drivers from countries such as Turkey, Jordan, Egypt and Kuwait. Several of those captives have been beheaded when the kidnappers' demands were not met.
"We're still unable to confirm if there's any missing but that's because we didn't know they were there in the first place"
At least four Westerners are also being held - two male French journalists and two female aid workers from Italy.
A group in Samarra, north of Dujail, claims to be holding two Australians and two East Asians but has provided no proof.
Australia has dispatched a special team to investigate although it said on Wednesday it had accounted for all 225 Australians known to be in Iraq.
"We're still unable to confirm if there's any missing but that's because we didn't know they were there in the first place," a Foreign Affairs spokesman in Canberra said.