Their vehicle was raked with gunfire as they returned home from work at a children's community center run by Pakistani charity Ujala, or Light, said district police officer Abdur Rashid Khan. Their driver was seriously wounded in the attack.
The shooting in Swabi district, about 75 km (45 miles) northwest of the capital of Islamabad, was the first attack on aid workers in the area.
The victims worked at the center for aid agency Support With Working Solutions, whose head Javed Akhtar said they had told their other 160 staff to suspend work following the killings.
The organization is involved in health education in underdeveloped parts of the country, Akhtar said. It had run a school and dispensary in Swabi and helped vaccinate children against polio, a disease that can cripple or kill within hours of infection.
He suspected the shootings might be linked to a string of attacks on polio vaccinators last month.
"This seemed to be part of the campaign against the polio drive by certain anti-polio elements," he said.
Two weeks ago, gunmen killed nine health workers taking part in a national polio vaccination drive in a series of attacks. Most of the victims were young women earning about $2 a day.
The Taliban said they did not carry out those attacks although its leaders have repeatedly denounced the vaccination program as a plot to sterilize people or spy on Muslims.
Aid workers have frequently been kidnapped or killed in Pakistan, a nuclear-armed nation of 180 million that is struggling to contain a Taliban insurgency and plagued by endemic corruption and violent crime.
(Additional reporting by Katharine Houreld; Editing by Louise Ireland and Giles Elgood)