Mohsen Mirdamadi, head of the Participation Front, the main reformist party, former deputy interior minister Mostafa Tajzadeh and Davoud Soleimani were found guilty of harming national security and propaganda against the regime.
The verdicts can be appealed.
Thousands of people have been detained during opposition demonstrations against last June's disputed re-election of hardline President Mahmoud Ahmadinejad, some of whom have been sentenced to stiff prison terms.
According to Iranian media reports, 10 protesters charged with taking part in the post-election unrest have been sentenced to death.
On January 28, Iran executed Mohammad Reza Ali Zamani and Arash Rahmani Pour, two members of a monarchist group. The timing of their arrests remains unclear, but they went on trial along with several post-election protesters.
