The special rapporteurs, who are part of the largest body of independent experts in the UN Human Rights system, said in a joint statement:
"We are deeply concerned by the long jail sentences imposed at a previous hearing on Pastor Victor Bet Tamraz, Amin Afshar Naderi and Hadi Asgari for allegedly 'conducting evangelism' and 'illegal house church activities', and similar charges."
The four rapporteurs, Mr. Ahmed Shaheed, Ms. Asma Jahangir, Fernand de Varennes, and Mr. Dainius Pūras are part of the Special Procedures of the Human Rights Council.
They added: "We strongly call on the Government to ensure that the final review hearing on Sunday is fair and transparent, in accordance with Iran's obligations under international human rights law."
The three Christians were initially given prison sentences of 10 years at a hearing in July. Mr Naderi received an additional five years for blasphemy.
They added that they are concerned with the lack of healthcare available to the men as well as the worry that this is not an isolated incident: "We are aware of several other reported cases in which members of the Christian minority have received heavy sentences after being charged with 'threatening national security', either for converting people or for attending house churches," they said.
They went on to speak of a disturbing pattern in the direction of persecution because of one's religion.
"We also urge the Government to immediately and unconditionally release all those who have been arrested and detained for exercising their right to freedom of religion or belief."
The UN experts notified the Government of Iran about their concerns.
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