The detained U.S. citizens were identified as Amir Hekmati, Saeed Abedini, and Jason Rezaian.
Hekmati, a former U.S. Marine, wrote to Secretary of State John Kerry a year ago, insisting that he was under duress when he confessed to spying on Iran. He was arrested in 2011 while visiting his grandmother in Iran.
On Friday, Kerry acknowledged that "Today marks the three-year anniversary of U.S. citizen Amir Hekmati's detention on false espionage charges while visiting his family in Iran. Mr. Hekmati is the eldest son; he has long been separated from his family and they need him home."
In another month, Kerry noted, Christian pastor Saeed Abedini "will have been detained for two years in Iran, on charges related to his religious beliefs."
Abedini's wife, Naghmeh, testified before a House subcommittee on foreign affairs in an attempt to gain his release, telling lawmakers, "I feel my husband has been abandoned."
Kerry commended Nagmeh, who he said "has spoken eloquently about the difficulties her family has faced during this challenging time."
Detained Washington Post reporter Jason Rezaian was one of three American journalists detained a month ago by Iranian authorities for unknown reasons. He and his wife, Yeganeh Salehi, an Iranian citizen and writer for the United Arab Emirates newspaper National, are still in detention.
The Committee to Protect Journalists marked the one month anniversary of their detention in a Twitter post on August 22.
Kerry contrasted the reporter's detention "in an unknown location" with Rezaian's "love of Iran [that] is seen in his reporting -- portraits of the generosity and kindness of the Iranian people."
And finally, the U.S. appealed to Iran to help find missing retired FBI Agent Robert Levinson, who vanished in Iran in 2007 and has since become one of the longest held Americans in history.
"Mr. Levinson went missing in March 2007 on Kish Island. His family has endured years of painful separation and worry," said Kerry, adding, "We are immensely concerned about his well-being and whereabouts."