The men - named as Kevin Gonot, Leonard Lopez and Salim Machou by AFP news agency - have 30 days to appeal.
They were among 12 French citizens captured in Syria by US-backed fighters. In February they were transferred to Iraq for trial.
The three are the first IS suspects from France to be sentenced to death.
France has yet to react to Sunday's court ruling in Baghdad. But when pressed on the issue in February, French President Emmanuel Macron declined to comment, saying it was a sovereign matter for Iraq.
Human rights groups have heavily criticised the trials of suspected IS fighters in Iraq. Activists they say the courts often rely on circumstantial evidence or confessions made under duress.
Gonot, 32, is from south-eastern France. He is believed to have entered Syria through Turkey to join the al-Nusra Front, a branch of al-Qaeda, before pledging allegiance to IS.
He was arrested in Syria with his mother, his wife, and his half-brother, AFP reports. A French court has also sentenced him in absentia to nine years in prison.
Machou, 41, belonged to an IS cell composed of European fighters that has carried out attacks in Iraq and Syria and planned others in Paris and Brussels, according to the Centre d'Analyse du Terrorisme (CAT), a French think tank.
Lopez, a 32-year-old from Paris, also travelled with his wife and two children to IS-held Mosul in northern Iraq before entering Syria, CAT quotes French investigators as saying.