The death sentences can be appealed, ISNA reported, quoting Bashiri Rad, director of public relations of the Tehran judiciary.
No other information was released about the three defendants or the charges they faced.
The Iranian government restricts the activity of journalists in Iran, making it difficult for news organizations to independently obtain or confirm information.
Tens of thousands of Iranians took to the streets in protest following the country's contested June 12 presidential election. Incumbent President Mahmoud Ahmadinejad won a second term despite widespread fraud accusations by supporters of opposition candidates.
Reformist former President Mohammad Khatami condemned "the atrocities committed at certain detention centers, as well as those committed during arrests, which, in some cases, resulted in murder," according to a statement issued by his office in August.
The Iranian government initially denied such accusations. But state media reported in August that authorities had fired and arrested the chief of an Iranian jail that was closed amid allegations of mistreatment of detainees.