In a statement, the Kilusang Larangang Gerilya-Kanlurang Bulakan-Pampanga of the New People’s Army (NPA) said the assault was meant to punish the members of the Maritime Group for their alleged "blood debts" to the fisherfolk of Hagonoy.
The policemen, the NPA unit claimed, have allegedly been extorting money from small fishermen and protecting big fishpond operators.
It added that the raid was a "major boost to the revolutionary struggle" in coastal areas of the province.
The firearms seized during the attack, it said, would be used to launch more attacks against "enemies of the people."
The rebels carted away eight M-16 rifles, a caliber .45 pistol and a caliber .38 revolver.
Eight guerrillas clad in police uniforms managed to enter the Maritime Group detachment premises by identifying themselves as augmentation troops from the regional Special Action Force.
Once inside, they opened fire at the lawmen, killing three — SPO3 Leonardo Paulino, PO1 Teodorico Collado and SPO2 Norberto Dinglasan — and wounding another, PO3 Dominador Perlas, who was hit in the shoulder.
The rebels arrived on board a Mitsubishi Adventure van but escaped aboard two motorized bancas waiting for them at the riverbank. The attack reportedly took no more than 10 minutes.
Police are still in hot pursuit of the attackers.
The Maritime Group patrols waterways in the province, along with a Riverine Company of the Army Special Forces based in Barangay Hangga, also in Hagonoy.