Siamak Namazi, Emad Shargi and Morad Tahbaz, who also has a British passport, were released alongside a fourth unidentified man, a lawyer for one of the men said.
A fifth American had already been released, a US national security official said.
Talks had taken place between Iran and the US to secure their release.
“While this is an encouraging step, these US citizens… should have never been detained in the first place,” said National Security Council spokesperson Adrienne Watson.
“Of course, we will not rest until they are all back home in the United States,” she added.
The prisoner release talks have been linked to negotiations over Iranian funds worth $6bn (£4.7bn) which had been frozen in South Korea, US media report.
Four of the dual nationals had been visually confirmed as having left Tehran’s notorious Evin prison, Jared Genser – a lawyer representing Mr Namazi – said.
Mr Namazi’s brother Babak said: “While this is a positive change, we will not rest until Siamak and others are back home; we continue to count the days until this can happen.”
The 51-year-old was first arrested in 2015 and was sentenced to 10 years in prison on security charges.
The sister of Mr Shargi, 58, said she “has faith in the work of President Biden and government officials” to secure his release following his detention in April 2018.
Mr Tahbaz, a 67-year-old businessman and wildlife conservationist, was first arrested during a crackdown on environmental activists in January 2018.
Iran has detained and imprisoned a number of dual nationals in recent years, usually on national security charges.
State Department spokesman Matthew Miller said two of the five individuals released “wish to remain private”.
He continued: “We are in touch with the families of US citizens involved, and we continue to monitor these individuals’ health and welfare closely.
Negotiations have been taking place for years to try to secure their release as part of a prisoner swap with Iranians now incarcerated in American and other western prisons, as well as the easing of US sanctions on Iran.
Citing sources close to the talks, Reuters said it could still take several weeks for the US citizens to leave Iran.
The New York Times reported that Iranian prisoners in US jails could also be freed as part of the deal, but the White House has not confirmed any details of what it offered to Tehran in order to secure the prisoners’ release.
