Motorcycle-riding gunmen opened fire on ANP workers in Killi Karbala village of Pishin district in which two were killed instantly while another suffered serious wounds, police said.
"Two of the workers died on the spot while the third one expired in hospital," a police official said.
"We arrested one suspect from the spot and he has been shifted to Pishin town for interrogation," he said.
No group claimed responsibility for the attack.
This incident came hours after a low intensity blast during National Party's public meeting in Turbat in southern Balochistan.
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A rally of the National Party chief Dr Abdul Malik Baloch was underway in Turbat when the blast took place.
The banned Pakistani Taliban, which is active in Balochistan, has threatened it will target ANP leaders and workers in the run up to the May 11 general election.
In another incident, unknown men threw an explosive device at the residence of an election commissioner in Kharaan town.
Levies officials said that militants on a motorcycle hurled a hand-held bomb at the residence of Muhammad Hashim Baloch, an election officer at Kharan bazaar.