The three deaths happened in separate incidents in the central province of Babil, said Dr. Ahmed Sabbah, director of the Institute of Forensic Medicine in the provincial capital Hilla.
The dead were aged 16, 22 and 30 and one of them was a woman, said Dr. Sabbah.
They had all gone outside to switch off the main supply to their homes during Wednesday’s storm, for fear that power fluctuations would damage their household electrical appliances.
The storm brought disruption to large parts of Iraq, not just Babil province, south of Baghdad.
Prime Minister Mohammed Shia Al-Sudani ordered civil servants to stay home nationwide on Thursday, except for key staff and security personnel.
In the northern Kurdistan region, the rains sent a river of mud pouring through the village of Khabat, with several shops suffering water damage, an AFP photographer reported.