The attack occurred in the main market of Mach town, located 70 km from the provincial capital of Quetta.
Police said the unidentified attackers, who were riding a motorcycle, fired indiscriminately at a shop and fled.
Five Hazara Shias were seriously injured.
Police took them to the Civil Hospital, where three men succumbed to their injuries.
Police conducted raids at several places to trace the attackers but did not make any arrests.
No group claimed responsibility for the attack though such incidents are usually blamed on the banned Lashkar-e-Jhangvi.
Human Rights Watch recently said over 100 Shias had been killed in sectarian attacks in Balochistan this year.
