Hitman Tencho Andonov and fixer Deyan Nikolov were also convicted – along with Mo Siddique – of playing key roles in the shooting of Toby Siddique, 38.
Andonov, the man who pulled the trigger in a flat in Glenrothes, Fife, was also convicted of attempting to murder former security man David Dalgleish, 44.
After one of the longest murder trials in recent Scottish history, a jury at the High Court in Edinburgh returned their verdicts after more than 11 hours of deliberation over three days.
The verdicts were greeted with gasps and wails from members of the Siddique family.
Delivery man Andonov, 28, and accountancy student Nikolov, 27, came to Britain from Bulgaria in 2007. Nikolov also worked part-time as a doorman at the Blue Mondays night club in Kirkcaldy, run by Mo Siddique, 42, of Glenfoyle House, Auchtertool.
All three had denied the charges against them, claiming Toby Siddique had been gunned down in a row with Eastern European criminals over contraband cigarettes.
There were even claims that Mr Dalgleish was to blame for the assassination.
But the trial also heard about years of feuding between the Siddique brothers over their tangled business interests, especially their property company which let out hundreds of homes in Glenrothes and Kirkcaldy.
When Toby Siddique died with two bullets in his head on the floor of a flat in Glenrothes, on 24 October, 2010, he was just hours away from gaining control over Moncreiff Properties. Mo Siddique and Nilolov were found guilty, by majority, of murder. In Andonov’s case the verdict was unanimous. Andonov was also found guilty of attempted murder.
Judge Lord Kinclaven told them the verdicts meant life sentences but he needed background reports before deciding the minimum each man must serve before becoming eligible for parole.
David Green, area procurator fiscal for Fife, described the murder as a “despicable crime”.
He said: “This is a crime motivated by greed. A tragic case of brother turning against brother and of a price being put on the victim’s life.
“No violent act should go unpunished and no-one who kills for reward should be allowed to escape justice. Andonov, Siddique and Nikolov have rightly been found guilty for their despicable crimes.”
The investigation into the attack was the largest and most complex in Fife Constabulary’s 62-year history at one stage involving 80 officers, and costing £1.4 million.
Detective Superintendent Colin Beattie said: “This was a large and extremely complicated investigation where the identity of the perpetrators was not immediately known.
“This required our staff to engage and work with other law enforcement agencies and partner organisations across the United Kingdom and beyond.
“I would like to thank the diverse range of communities in Fife for their help and support in solving this case.”
“Various nationalities and cultures cut across the entire investigation and our staff received cooperation and assistance from all concerned.”
The three men will remain in custody pending sentence in Perth on 28 March.