Anton Duffy, 39, Martin Hughes, 36, and Paul Sands, 32, were convicted in July of conspiring to kill former Ulster Defence Association (UDA) chief Adair and his right-hand man, Sam McCrory.
Following a nine-week trial at the High Court in Glasgow, Duffy and Hughes were also convicted of terrorism charges.
Duffy, the “instigator and driving force” behind the plot, was jailed for 17 years when he returned to the court for sentencing. He will also be supervised for a further three years following his release, under the terms of an extended sentence imposed by judge Lady Scott.
Hughes was sentenced to 11 years’ imprisonment, while Sands given an extended sentence, involving ten years behind bars and supervision for a further three years after release.
Two other men found guilty of organised crime charges were also jailed at the same hearing.
Craig Convery, 37, was jailed for nine years while his associate, Gordon Brown, 30, was given a six-year jail term.
Duffy, Sands and Hughes were arrested in 2013 after two Police Scotland investigations into terrorism and organised crime.
Duffy, from Donegal, was the ringleader of an unaffiliated active service unit inspired by dissident republicanism that planned to carry out the double murder with Sands and Hughes while on home leave from prison, according to police.
Operation Hairsplitter was set up late in September 2012 to investigate an attempt to procure firearms, including an AK-47, by the gang led by Duffy.
MI5 and Police Scotland carried out investigations throughout 2013, including using covert listening devices to gather information.
Adair was a leading figure in the UDA during the Troubles in Northern Ireland and moved to Scotland after being released from prison as part of the Good Friday Agreement.
Speaking outside court, Adair said the convicted men “deserved every minute” of the sentences they were given
“The severity of the sentence reflects the seriousness of the charges and I’m just delighted that justice has been done today,” he said.
Addressing all three men involved in the plot to kill, Lady Scott said: “This was a conspiracy to murder two men. It seems clear you callously intended to shoot or execute your intended victim, after confronting him by surprise.
“This murderous conspiracy was long thought of and discussed by Mr Duffy whilst in prison and on leave and it involved considerable planning by all involved.”
Detective Chief Superintendent John Cuddihy, of Police Scotland, said: “Make no mistake, the intent of those who have been sentenced today was to carry out a murderous terrorist attack on the streets of Scotland.”