Khanshin district Chief Shah Mahmood Mir said the victims of the Tuesday accident were all from the same family.
The three injured were in critical condition and were taken to a hospital for treatment, he said.
No group claimed responsibility for the blast, Khaama Press reported Wednesday.
Local officials suspect Taliban militants are responsible for the attack because they frequently use improvised explosive devices in the area to target Afghan and coalition security forces.
Susan Sarandon 'very excited' about daughter's pregnancy
Teacher apologizes for showing sexual image of herself in class