The three Muslims believed Islam was under attack opposed Australia's military involvement in Afghanistan, the prosecutor told the Victorian Supreme Court during their trial in Melbourne.
Two other Muslim men were found not guilty.
Australia has around 1,550 troops in Afghanistan, based mainly at Tirin Kot in southern Uruzgan province, and is the largest non-NATO member of the international coalition fighting Taliban insurgents in the country.
Wissam Fattal, 34, Saney Aweys, 27, and Nayef El Sayed, 26, were found guilty of conspiring to prepare for or plan a terrorist act between February 1, 2009 and August 4, 2009.
As the jury left the court following the verdict, Fattal said: "Islam is truth religion. Thank you very much", Australian Associated Press reported from the courtroom.
Australia's biggest terror trial ended in February 2009, when Muslim cleric Abdul Nacer Benbrika was jailed for 15 years for leading a cell that planned to bomb a 2005 football match in Melbourne. Altogether, 12 people were jailed over the plot.
Australia has not suffered a peacetime attack on home soil since a bombing outside a Sydney hotel during a Commonwealth meeting in 1978 that killed three people. But 95 Australians have been killed in bomb attacks in Indonesia since 2002.
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