The police van was parked near the Darum Uloom religious seminary in Korangi.
"Two policemen died on the spot while another one died in a hospital. A 12-year old boy was also killed as he was caught in the firing when he was passing by near the seminary," senior police official Arif Aslam said.
A large number of police and paramilitary troops rushed to the area after the incident and cordoned off the crime scene.
On May, two policemen were gunned down by unidentified men in the posh Bahadurabad area.
Senior Superintendent of Police (SSP) of the counter terrorism department in Sindh Raja Umar Khatab said that 9 mm shells were found from the spot which indicated same group was involved which had earlier carried out the attacks on the policemen in Site area and Bahadurabad.