They are the first French IS members that received death sentences in Iraq. The hearings push forward the trials of hundreds of people — many of them foreigners — captured after the militant group lost its final stronghold in Syria last year.
The three sentenced on Sunday, Kevin Gonot, Leonard Lopez and Salim Machou, are among 12 French citizens who were caught in Syria by U.S.-backed Syrian Democratic Forces. They were transferred to Iraqi custody in February.
They can appeal their death sentence.
Last Monday, another French man and a German woman were sentenced to life in prison in Iraq for belonging to IS.
Nadia Rainer Hermann, a 22-year-old German national, and Lahcen Ammar Gueboudj, who reports say is on his 50s, pleaded not guilty to joining the Islamist group that waged a campaign of terror in Syria in 2014.
Gueboudj and Hermann were tried individually, but their sentencing was with another 13 people tried on Monday in crowded small courtroom.
Human Rights Watch and other rights groups denounced Iraq's anti-terror trials. They say Iraqi courts generally depend on circumstantial evidence or confessions obtained under torture.
According to an Amnesty International report in April, Iraq remains in the top five "executioner" nations in the world.