The civilian driver of the vehicle was also killed in the attack on Tuesday, said Qaisar Hamid of Baquba General Hospital. Two others were wounded, he said.
Assailants fired at the vehicle with machine guns.
The police officers were from out of town and were due to travel to Jordan for training on Tuesday.
They reported to the police headquarters in Baquba, but were driving away after being told the journey to Jordan had been postponed for a day, when they were hit.
US patrol hit
Meanwhile, armed assailants also opened fire on a US army patrol near the northern Iraqi city of Mosul on Tuesday, killing one soldier and wounding five, the US military said.
A civilian driver of the vehicle
also died in the attack
The five wounded soldiers were evacuated to military hospitals in Mosul and Baghdad.
Their patrol came under small arms fire as it was travelling on the western side of Mosul, the military said in a statement.
The death followed the killing of two soldiers in Baghdad on Monday.
A loud explosion also rocked al-Muthanna airport in central Baghdad, sources told Aljazeera.
They said columns of smoke were seen rising from the site. A US source confirmed the explosion.