The group from Azerbaijan, detained on 24th June 2016 after going to the ceremony in capital city Tehran, were also denied help from lawyers or embassy staff.
Chief Executive of anti-persecution charity Christian Solidarity Worldwide (CSW), an organisation working for religious freedom, Mervyn Thomas said: "It is unacceptable that visitors were arrested simply for attending a Christian wedding.
"This constant intimidation and harassment should not be occurring in a country with a constitution that not only recognises Christianity, but also states that no-one should be molested or taken to task simply for holding a religious belief."
The trio, 51 year old Yusif Farhadov, 48 year old Eldar Gurbanov and 37 year old Bahram Nasibov were eventually charged with "conducting activities against national security" but have now been bailed.
They were kept at Evin Prison in northwest Tehran (pictured).
They were released on 29th October 2016 after the full amount for bail was paid and were given permission to return home to Azerbaijan on Monday.
Mervyn Thomas added: "We urge the Iranian authorities to end the continuing infringement on the right to freedom of religion or belief.
"We also call on Iran to ensure that the nation's practices, legal procedures and provisions do not contradict its constitutional and international commitments to ensure the full enjoyment of this right for every religious community."