Two men and a woman are being questioned about the murder of Pc Stephen Carroll in Craigavon.
Pc Carroll, 48, was gunned down as he answered a call-out in Co Armagh town last March.
He was married and lived in Banbridge, Co Down.
The Continuity IRA claimed responsibility for his death.
On Tuesday, detectives from the PSNI's serious crime branch detained a 40-year-old man in Lurgan, Co Armagh.
A man aged 36 and woman aged 37 were arrested in Craigavon. They are being questioned at Antrim serious crime suite.
A teenager and a 38-year-old man have previously been remanded in custody charged with the murder.
Pc Carroll was killed two days after the Real IRA shot dead two soldiers outside Massereene Barracks in Antrim.
© Press Association