Captured in Syria by a US-backed force fighting the jihadists, they are the first French Isis members to receive death sentences in Iraq, where they were transferred for trial.
Named as Kevin Gonot, Leonard Lopez and Salim Machou, they have 30 days to appeal.
Iraq has taken custody of thousands of jihadists repatriated in recent months from neighbouring Syria, where they were caught by the US-backed Syrian Democratic Forces during the battle to destroy the Isis “caliphate”.
Iraqi courts have placed on trial hundreds of foreigners, condemning many to life in prison and others to death, although no foreign IS members have yet been executed.
Those sentenced on Sunday were among 12 French citizens who were caught in Syria and transferred to Iraqi custody in February.
Rights groups including Human Rights Watch have criticised Iraq's anti-terror trials, which they say often rely on circumstantial evidence or confessions obtained under torture.
The country remains in the top five “executioner” nations in the world, according to an Amnesty International report in April.
Analysts have also warned that prisons in Iraq have in the past acted as “academies” for future jihadists, including Isis supremo Abu Bakr al-Baghdadi.