Mohammed Istiak Alamgir (second right), Yousaf Bashir and Rajib Khan (back, third right) gave speeches at a series of meetings in Luton during Ramadan last year, attended by up to 80 people including young children. They are all from the town.
Mr Khan was also convicted by a majority of nine jurors to one at the Old Bailey of arranging one of the meetings at a local church.
The charges:
Mr Alamgir, 37, was found guilty of three counts of addressing meetings to encourage support for a proscribed organisation, namely IS, or to further its activities.
Mr Bashir, 36, was found guilty by a majority of addressing a similar meeting on June 29.
Mr Khan, 38, was convicted of arranging, managing or assisting in arranging a meeting to support IS on July 11, and addressing a similar meeting on July 11. They are all from Luton.
The jury of 10 was unable to reach verdicts on two co-defendants - Mohammed Choudry and Zaiur Rahman - who will face a retrial at the Old Bailey on November 14.
Rahman, 38, from Luton, faces three counts of arranging a meeting to support IS while Choudry, 23, from Maidenhead, Berkshire, is charged with a single count of encouraging support for IS.
Commander Dean Haydon, head of Scotland Yard's Counter-Terrorism Command, said: "The biggest concern we had was with children present of a very young age, as people start growing up, particularly in Luton, we need to avoid individuals either travelling to Syria or becoming involved in extremist acts or even terrorist acts.
"What this group of people were doing was infecting the young minds of children.
"Speeches like theirs inspire the terrorists of tomorrow and I am immensely pleased with the excellent work of my officers and Bedfordshire Police, which ensured the conviction of these men."
