The verdicts could still be revised, he added.
The death sentences have been the subject of widespread international condemnation, criticism Ismaili rejected.
The three suspects were violent ringleaders, he said, and had set fire to a number of buildings and transport facilities.
The men had recorded their actions on their phones, Ismaili added, and the evidence was considered by the court.
Iran was rocked by days of unrest following a hike in petrol prices in November, 2019.
The protests were violently suppressed by security forces.
Iran’s government referred to the demonstrators as paid mercenaries of its arch enemies, the United States, Israel and Saudi Arabia and claimed the protesters’ true aim was to weaken or even bring down the government.
Iranian authorities have still not released reliable information on the numbers of fatalities during the unrest.
Unconfirmed reports put the number of dead at 200, including demonstrators and police. Foreign sources put the number even higher, although neither estimate can be verified.
More than 1,000 demonstrators were arrested at the time.