The three men, all in their 30s, were detained yesterday at Gurranabraher and Bridewell Garda Stations for questioning about the murder of Mikolaj Wilk in Ballincollig last summer.
Two of the men were released overnight and a file is being prepared for the Director of Public Prosecutions. The third man remained in custody today. He was arrested under Section 50 of the Criminal Justice Act 2007 and can be held for seven days.
Mr Wilk, aged 35, was brutally murdered in front of his wife and kids by a masked gang at his home at Maglin Bridge, near Ballincollig, on June 10, 2018.
It was one of the most grotesque murders Cork has ever seen and up to 15 gardaí who attended the scene required trauma counselling afterwards.
The attackers burst into the house at around 3am and hacked Mr Wilk to death before fleeing the scene.