The three were undergoing treatment at the Pariyaram Medical College Hospital here, police said.
Thirtyfive people were injured when the LPG tanker overturned and exploded after hitting a road divider on August 27. Six houses, 15 shops and some vehicles were gutted in the resulting huge fire.
Two of the injured died on Tuesday while two others succumbed to the burns yesterday.
The condition of 15 of the injured, admitted to various hospitals, was critical, police said.