Officials blamed the attack on Balochistan separatist insurgents who are fighting for political autonomy and a greater share of profits from the region's natural resources.
One of the three rockets landed in a square in Quetta, causing all the fatalities and injuries, Daud Juneju, the city police chief told AFP.
"Both the traffic officers were on duty, two passers-by were also killed," he said, adding that all those injured were civilians.
Two other rockets damaged a house but caused no casualties, he said.
Hamid Shakil, another police official confirmed the incident.
Intelligence officials said Baloch separatists were involved in the attack.
Balochistan, which borders Afghanistan and Iran, has seen a surge in violence recently, with the province suffering from a separatist insurgency, sectarian violence and Taliban militants.
Hundreds of people have died since the separatist rebellion broke out in 2004.
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