The trio were killed overnight by US troops near Salman Pak south of the northern Iraqi city of Kirkuk.
"The police had a roadblock on the road linking Kirkuk and Baghdad. An America patrol arrived around 0200 (2300 GMT Friday) and opened fire, taking the police to be guerrillas," said Second Lieutenant Salam Zankana on Saturday.
The three dead men were identified as Ahmed Husain, Abd al-Rahman Salih and Saqr Naji Husain.
The US army said it was investigating the incident.
In the southern city of Najaf a middle-ranking district mayor was killed in a hail of submachine gunfire and a provincial party official was left in critical condition with her 5-year-old son dead.
It was the third attack in days on former officials of the Baath Party.
Ali Qasim al-Tamimi , district mayor of al-Furat district, died Friday night when gunmen fired on him as he was shopping with a friend in downtown Najaf, according to Lt. Raed Jawad Abdel Saada.
Saturday morning, two assailants riding by on a bicycle opened fire on former provincial party official Damia Abbas and her son as they were leaving their home.
The five-year-old boy was killed instantly, and Abbas was hospitalised in critical condition, according to another police officer, Lieutenant Raid Abbas.
Damia Abbas was believed to have participated in putting down a 1991 Shia uprising against the government of Saddam, who violently repressed Iraq's Shia majority.
Al-Tamimi's position would have involved him acting as an informer, reporting to Baath Party officials in Baghdad on the political activities and jobs of residents.
On Wednesday, another Baath party official accused in the 1991 repression was lynched by an angry crowd and killed in Najaf, which is 160 kilometres (100 miles) southwest of Baghdad.
Aznar in Iraq
Women and children have been
victims of the anti-Saddam backlash
News of the deaths broke as Spanish Prime Minister Jose Maria Aznar paid a surprise visit to Iraq on Saturday, Spain's national radio reported.
Aznar flew in with his defence minister to inspect Spanish troops deployed in the south of the country.
Aznar left Madrid on Friday for Kuwait from where he went by helicopter to Diwaniyah, the radio's special correspondent said.
Aznar became the third top western politician - after US President George W Bush and Secretary of Defence Donald Rumsfeld - to travel to the war-torn country in three weeks.
Aznar was a staunch backer of the US-led invasion of Iraq, and contributed some 1,300 troops despite virulent public opposition to the attack without a UN mandate.
A delegation of 17 people, among them the Spanish Defence Minister Federico Trillo, accompanied Aznar for his several-hour visit to the city 160 kilometres southeast of Baghdad.
The prime minister was to lunch with Spanish soldiers and meet with local authorities before flying back to Madrid on Saturday afternoon.
On December 12, the Spanish government decided to prolong the stay of its troops by six months.
Spain has lost 10 men since it deployed soldiers and intelligence officers in Iraq.