Judges at the Court of Appeal rejected claims that Stephen McCaughey and brothers Ian and Jason Weir received unduly lenient jail terms for their roles in the killing of Philip Strickland.
Mr Strickland was killed with a shotgun during a dispute in 2012. He was found dead in his car on the Ballydrain Road, near Comber.
The killing followed a row with farmer Jimmy Seales, the father of the Weir brothers.
Seales, 57, is serving a minimum 15-year prison term after being convicted of carrying out the murder.
McCaughey, 27, from Shackelton Walk in Newtownards, was also found guilty and ordered to serve at least 10 years behind bars.
Ian and Jason Weir, of Derryboye Road and Raffery Road, near Killinchy, had earlier pleaded guilty to murder.
During the trial, Ian Weir, 29, placed his father at the scene of the shooting, armed with a shotgun.
He received a minimum prison term of four years, while his 28-year-old brother was told he would spend at least nine and a half years in jail.
The discounted sentences were referred back to the Court of Appeal.
Senior counsel for the DPP argued all three should have been handed longer jail terms.
But senior judges ruled that none of the sentences were unduly lenient.
Lord Justice Gillen said the level of co-operation provided by Ian Weir should not be underestimated, as he gave evidence against his father, "a man who had bullied and dominated him and his brother for many years".
He said: "Such evidence against a close family member is invaluable and not easily obtained."
He also held that Jason Weir played a secondary role in the killing, compared to his father.
McCaughey, the judge said, became involved out of a "foolish, mistaken sense of loyalty" to one of the Weirs, with no evidence of him carrying out any physical attack.
The Director of Public Prosecutions (DPP) has failed in a bid to have three murderers' prison sentences increased.