The soldiers from the Frontier Corps were hit in the head while patrolling on foot in Sabzal, a neighborhood in west Quetta, capital of restive Balochistan province.
"Two of them died on the spot while the third succumbed to his injuries in hospital," Khan Wasay, a spokesman for the Frontier Corps told AFP.
Wasay said the motive for the attack was unclear, but Quetta was already on alert for the Muslim holy month of Muharram, marking the anniversary of the martyrdom of Imam Hussein (AS), the grandson of the Prophet Muhammed (PBUH).
Abdul Razzaq Cheema, a senior police official, in Quetta confirmed the attack and casualties.
No group has yet claimed responsibility but Baloch separatists demanding greater autonomy have been waging an insurgency for years, and the province is also riven by sectarian strife and violence.
Separately, four people including a soldier from the Frontier Corps were wounded on Friday after intercepting a suicide bomber on the Pakistan-Afghanistan border in the northwestern Kurram tribal district.
Security officials said the bomber was trying to cross into Pakistan at the Kharlachi Gate, but "his attempt to create a big disaster was successfully foiled".