Violent protests have erupted in several parts of Baluchistan since a Baluch nationalist rebel leader was killed in fighting with security forces on Saturday.
"Three people were killed on the spot while another three are seriously injured," Maqsood Anwar, a police officer in the town of Hub, told Reuters on Tuesday.
Five or six other people had minor wounds, he said.
Shoaib Nausherwani, provincial interior minister, said the blast went off in a roadside hotel in Hub.
He said it was too early to say if the incident was linked to the protests over the weekend killing of Nawab Akbar Khan Bugti, a veteran Baluch politician.
Rebels fighting for autonomy and a greater share of the profits from Baluchistan's gas have been waging a low-intensity insurgency for years.
But they have stepped up their attacks over the past year and have been blamed for several deadly bomb attacks.