"There are also major concerns about the way the recent trials of opposition activists were conducted, and I hope these judgements will be reviewed carefully by the higher courts," said Navi Pillay, the UN High Commissioner for Human Rights, in a statement.
Under international rules that Iran has signed up to, the death penalty can only be applied in circumstances involving serious crimes and after fair trials, she pointed out.
"The imposition of the death penalty for crimes that do not result in loss of life is contrary to" the International Covenant on Civil and Political Rights, she said, also reiterating her opposition to the death penalty.
The Iranian justice ministry was quoted by the ISNA news agency on Saturday as saying that the three had been sentenced to death for their role in the "post-election incidents."
Pillay also said she was "deeply dismayed" that Iran had executed a man over a murder he committed when he was still a teenager.
"This latest execution shows there are no guarantees of clemency for juveniles until Iran changes its law and practice to end execution of juvenile offenders once and for all," said Pillay.
Behnoud Shojaie was hanged Sunday after being convicted of stabbing to death 17-year-old Ehsan Nasrollahi during a fight in August 2005 when he himself was aged 17, Iranian news agencies reported.
A UN human rights expert issued a separate statement denouncing the execution of Shojaie.
"The juvenile death penalty is a negation of the essential principles of juvenile justice accepted by all states, including Iran," said Philip Alston, UN Special Rapporteur on extrajudicial, arbitrary or summary executions.
Alston said he had written three letters to Tehran on the case.
In reply, Tehran said it was "doing their utmost" to get Shojaie's family to accept paying "blood money" in order to get the execution waived, Alson revealed.
"While such efforts to mediate between the family of the child found guilty of a killing and the victim?s family are welcome, they are utterly insufficient to satisfy Iran?s obligations under international law," he added.
"The stance taken by the Government in its correspondence with me, that they have no possibility to halt an execution if the murder victim?s family insists on it, is untenable as a matter of international law," he added.
