The police van was on a routine patrol in Kotli area of Hangu in Khyber-Pakhtunkhwa province when it was attacked at 9.30 am.
The attackers fired indiscriminately at the vehicle, killing three policemen instantly, the officials said.
Several injured policemen were taken to a nearby hospital.
Security forces cordoned off the area and launched a search to trace the attackers.
No group claimed responsibility for the attack though such incidents are usually blamed on the Pakistani Taliban.
