The bomb was planted on a motorbike parked along the road in Almo Chowk of Quetta city and it exploded when people were busy in shopping during the holy month of Ramadan.
"Three people were killed and 20 injured in the attack," a police official said.
The victims were shifted to the Civil Hospital Quetta, where some of them were said to be in critical condition.
Security personnel rushed to the place of attack and have started investigations, he said.