Failed showbusiness agent Lele Mora and television host Emilio Fede were sentenced to seven years each in jail, while showgirl-turned-politician Nicole Minetti was given five years behind bars.
Among the girls allegedly recruited by the trio for parties at billionaire Berlusconi's villas was Moroccan-born Karima El-Mahroug, a then 17-year-old exotic dancer nicknamed "Ruby the Heart Stealer".
Berlusconi was sentenced in a separate trial in June to seven years in jail for paying for sex with Ruby and abusing his power to hide the liaison - though the punishment was suspended while awaiting appeal.
The Milan court Friday banned Moro and Fede for life from holding public office and working with minors, while Minetti was given a five-year ban on holding public office. All three were ordered to pay court costs.
The judges also asked prosecutors to launch an investigation into Berlusconi, his lawyers Niccolo Ghedini and Piero Longo, and Ruby, for lying under oath while testifying in Mora, Fede and Minetti's defence.
Minetti, who is on holiday in Spain, told her lawyer by telephone that she was "stunned by the excessive punishment", which will also be suspended until the appeals process has been exhausted.
In his summing up speech in May, prosecutor Pietro Forno said the three had arranged "orgies" at Berlusconi's mansion and cited the tycoon's ex-wife Veronica Lario, who accused her then husband of consorting with "young virgins".
His colleague Antonio Sangermano said the three were like "tasters of fine wine" and had obtained financial advantages from Berlusconi because "they know all the secrets" of those nights.
"They carried out a sort of exam of the capacities of the young women and then injected them into the circuit of soirees," Sangermano said. Minetti was also an active participant and "performed sexual acts for money", he said.
All three denied the charges, insisting that while they may have invited girls to the premier's Milan villas, it was to attend nothing more than elegant dinner parties.
The guilty verdict will come as a blow for Berlusconi, who is facing a slew of legal troubles which have threatened to rattle the government.
On July 30th Italy's highest court is expected to begin preparing to rule on a conviction against him for tax fraud connected to his Mediaset empire.
Should it uphold the ruling, Berlusconi could see himself banned from holding public office and forced to retire from the political scene.
Political experts warn such a bitter blow could have wider repercussions on Italy's fragile grand coalition government, which relies on the support of the media magnate's centre-right party.
President Giorgio Napolitano made an explicit reference to Berlusconi's troubles on Thursday when he called for responsibility from all sides to ensure political stability in the eurozone's third largest economy.