Earlier, they tried to launch a campaign to raise "blood money" to save the life of a young man on death row for a crime allegedly committed when he was 17.
The three have started raising money to save Behnoud Shojaee, who is awaiting his execution.
On many occasions, rights activists have sought to stop such executions by negotiating with the victim's families, who under the mullahs' law have the right to pardon.
On Tuesday, the EU presidency called on Iran to "comply with its international commitments and to take the immediate measures required to grant a stay of execution to Mr. Shojaee and all other minors facing death sentences."
While the EU countries were appealing to the clerical regime to stay the execution of Shojaee, they hanged a 20-year-old man who was 15 at time of the alleged crime in the central city of Isfahan on Wednesday.
He was transferred to solitary confinement in preparation for his execution on August 18. However, when his family and attorney went to the prison on the next day, they were told that his execution has been stayed.
"Jubilant of the news, his family and I left the prison. But only a few hours latter I learned through the official site of Isfahan prosecutor's office that he was hanged," said his attorney Mohammad Mostafavi in total surprise.
According to rights groups, 114 youths face gallows for the crimes allegedly committed when they were minors. The youngest is a 13-year-old boy named Ahmad Nowroozi sentenced to death by the mullahs' judiciary three years ago in the southeastern province of Sistan and Baluchistan.