The first blast took place at a garbage dump in Landhi area of Karachi this evening.
The bomb was hidden in an empty container, witnesses said.
The second and more powerful bomb went off about 15 minutes later when policemen and rescue teams had gathered at the site.
Officials said two policemen, including Deputy Superintendent of Police Kamal Khan Mangan, a private security guard and a passer-by, were killed.
The second bomb, which was packed with ball bearings, was hidden in a cement block.
Over a dozen injured people, including rescue workers, were taken to a nearby hospital.
Five vehicles, including a police van and ambulances, were damaged.
No group claimed responsibility for the bombings, which occurred amidst a heightened alert across Pakistan on the eve of Eid-e-Miladun Nabi, the birth anniversary of Prophet Mohammed.
President Asif Ali Zardari and Prime Minister Raja Pervez Ashraf condemned the blasts.
Scores of people have been killed in incidents of sectarian and political violence in Karachi, Pakistan's largest city, since last year.
