The trio were among 13 French citizens handed over to Iraq in January by the US-backed Syrian Democratic Forces, officials said.
“They were sentenced to execution after it was proven that they were members of the terrorist Islamic State organisation,” one court official said.
“All three convicted Frenchmen rejected the ruling and asked to be tried in France, but judges ignored their request,” a court-appointed lawyer said.
Appeals have been made against the convictions.
Iraqi president Barham Saleh had said during a February visit to Paris the 13 would be prosecuted in accordance with Iraqi laws.
The French government has so far categorically refused to take back Isis fighters and their wives.
Foreign minister Jean-Yves Le Drian referred to them as “enemies” of the nation, saying they should face justice either in Syria or Iraq.
It was not immediately clear how France, which abolished the death penalty nearly four decades ago, will react to the sentencing of its citizens.
French authorities have repatriated a handful of children and plan to continue on a case-by-case basis.
Human rights groups and advocates of the men in France say it is not certain they committed crimes in Iraq, or if they were even ever in the country.
They also doubt the impartiality of the courts, which have handed down hundreds of death sentences to Iraqi suspects in trials that run for just a few minutes.
Iraq has detained or imprisoned at least 19,000 people accused of connections to Isis or other terror-related offences, and sentenced more than 3,000 of them to death, according to an analysis by the Associated Press last year.
Additional reporting by agencies