Public prosecutor Mousa Nouri was gunned down in the city of Zabol, in Sistan-Baluchestan province, along with his driver in November in a murder first claimed by a Sunni extremist group.
The three men convicted of his murder, Omir Piri, Alireza Dehmardeh and Iman Galavi "were sentenced to death for forming a terror team and killing... Nouri," IRNA news agency said.
They were executed "in public this morning" in the same spot where Nouri was shot dead "after the Supreme Court upheld the verdict," the agency said.
UN chief Ban Ki-moon in March voiced alarm over the high number of hangings in Iran, saying at least 500 people had been executed last year, including 57 in public.
The Sunni extremist group, Jaish-ul Adl (Army of Justice in Arabic), had claimed responsibility for Nouri's murder saying it was in retaliation for a mass hanging of Sunni rebels by Shiite-dominated Iran.
However, Iranian officials dismissed the claim, saying the attack was carried out by drug smugglers who held a grudge against the prosecutor.
Murder, rape, armed robbery, drug trafficking and adultery are among the crimes punishable by death in Iran, based on its interpretation of sharia law in force since its 1979 Islamic revolution.
