French foreign ministry spokesman Herve Ladsous said on Tuesday that the French nationals were in Iraq working for US companies, tasked with rebuilding infrastructure in and around Baghdad.
They were driving in a convoy near Falluja, west of the Iraqi capital, when their car broke down, diplomats said.
The occupants of a passing vehicle shot at the French men, killing two and wounding the third.
No further details were immediately available on the timing or circumstances of their deaths.
Foreign citizens attacked
"These tragic events lead us to recall, as posted on the foreign ministry's website, that it is urgent that all French nationals currently in Iraq make themselves known to our consular authorities and to the French interests section," Ladsous said.
The three French nationals in Falluja were not registered with French consular services in Iraq, he noted.
Several foreign citizens have been killed in attacks by resistance fighters opposed to US-led occupying forces that deposed former Iraqi president Saddam Hussein.
Two Japanese diplomats and seven Spanish intelligence officers were shot and killed in Iraq in separate attacks in November.