Acting on a tip-off, the forces raided a building in Hazar Ganji area of provincial capital Quetta in the early hours.
Three militants, including a suicide bomber and bomb- making expert, were arrested, a police officer said.
"They were shifted to a secret location for questioning," he said.
Police suspected that the militants were planning to attack the Hazara Shia minority who dominate the Hazar Ganji locality of the city.
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Suicide jackets and a large number of arms and ammunition were also recovered from their possession, the report said.
Balochistan has faced violence by Taliban-linked sectarian groups and Baloch nationalists.
The security forces have increased targeted operations against the militants after 150 people, mostly children, were killed in a Taliban attack on an army public school in Peshawar last month.