The Italian defence ministry said a fourth Italian was seriously wounded in Thursday's attack.
The Italian news agency Apcom, citing government sources, said more were wounded.
Romania confirmed that one of its soldiers was killed in the attack. It was Romania's first combat death of the Iraq conflict.
The bomb hit a four-vehicle convoy on its way to relieve troops at the Iraqi police station in the city of Nasiriyah.
Elsewhere, a US jet fired two missiles at insurgent positions in Ramadi, US officers said.
Fighting also broke out northeast of Baghdad between Iraqi forces and fighters, killing several police officers and civilians.
Police and residents said clashes also took place northeast of Baghdad after fighters attacked four police checkpoints in Baqouba.
In Ramadi, 115km west of Baghdad, US forces exchanged fire with armed groups who attacked with small arms and shoulder-fired rockets from a former train station and a nearby building.
Romania has announced the death
of its first soldier in Iraq
Lieutenant Colonel Ronald Clark, commander of the 1st Battalion, 506th Infantry Regiment, said a US jet fired two missiles at the buildings and US forces returned fire with mortars and rockets, killing eight of the attackers.
In a separate incident, one Iraqi army soldier was killed during a firefight in a nearby Ramadi neighbourhood, army officers said.
In Baghdad, a roadside bomb hit an Iraqi army patrol, killing one soldier, police said.