Salih al-Ageili, a member of the Sadr Movement's parliamentary bloc, became the third death in the blast, which security sources told the Kuwait News Agency was a bomb-laden motorcycle.
Ageili was being prepared to undergo surgery when he died, KUNA said.
The bomb exploded as Ageili's convoy was passing, KUNA reported. Three bodyguards and four bystanders were injured.
The Sadr Movement is affiliated with radical Shiite cleric Moqtada Sadr.