The three police officers died when their patrol vehicle struck a bomb planted along the road in the district of Bahrami Shahid in southern Gazni province, according to the province's deputy police chief, Asadullha Ensafi.
A fourth policeman was wounded in the attack.
There was no immediate claim of responsibility for the bombing but Mr Ensaf blamed the Taliban, who frequently target the country's security forces.
Separately, Nato said one of its service members was killed in an attack in the east. The US-led military alliance provided no other details.
The death brought to 51 the number of Nato service members who have died in Afghanistan so far this year, including 38 Americans.