Christians Behnam Irani, Matthias Haghnejad, and Silas Rabbani were denied access to their attorney ahead of their appeal hearing in Iran on Monday.
The men are only allowed to see their lawyer on the day of their hearings, and requests for earlier meetings have been denied.
Pastor Irani, Pastor Haghnejad, and Deacon Rabbani were leaders in the Church of Iran prior to their imprisonment, and have been charged with "action against national security" and "creating a network to overthrow the system," because of their Christian ministry.
Capital charges were dropped in October, but the men were sentenced to six years in prison. Irani, who has had multiple run-ins with Iranian authorities, was handed an additional six years.
Attorney Vahid Moshkani Farahani said he is particularly concerned for his client Pastor Irani, who has suffered health problems while incarcerated.
Human rights advocacy group Christian Solidarity Worldwide (CSW) condemned the men's sentences and their treatment by Iranian officials.
"It is unacceptable that these men have been denied access to their legal counsel ahead of the court hearing," CSW's Chief Executive Mervyn Thomas said in a statement. "We remain concerned about the lack of due process in their case and the exceptional nature of their punishment. They have committed no crime and are effectively being punished for their faith.
"We are deeply concerned for their wellbeing and in particular for Pastor Irani's health, which has deteriorated significantly during his unjust detention," he added. "We urge the Iranian authorities to release these men and the many others who are being held in contravention of the international covenants guaranteeing freedom of religion or belief to which Iran is party."
The appeal hearing was held in the Sixth Branch of Karaj Revolutionary Court on Monday, but the results of the trial are not yet known.
The Christians are being held in remote locations away from their families, with Haghnejad and Rabbani imprisoned in Minab in the Persian Gulf, and Pastor Irani imprisoned in the southern city of Zabol.