The trio, including, Minuwangoda Pradeshiya Sabha member Kelum Krishantha, stand accused of accumulating weapons for the underworld in the south.
The seized cache includes a 7.65 mm pistol, ammunition and 90 hand grenades.
Gampaha police had been tipped-off about a politician who is trafficking in weapons and drugs.
Investigations have revealed that an underworld leader, Keselbaddara Wasantha, had deposited Rs. 1.1 million in the PS member’s account to gather weapons for the gang led by Podi Lassie.