The court also handed life sentences to 50 other defendants, the official said, and jail terms of between five and 10 years to 10 more.
The incident at the centre of the trial took place on August 14, 2013 as security forces violently dispersed two protest camps in Cairo, in an operation following the military's ouster of president Mohamed Morsi that left more than 700 people dead.
Prosecutors said the defendants marched on Helwan police station in a southern suburb of Cairo and fired at police inside from behind barricades, killing three and wounding scores.
They were then alleged to have torched the building and 20 police vehicles parked nearby.
Sinai border attack: Thirteen Egyptian police killed
The verdicts -- which can be appealed -- also saw defendants ordered to pay some 2.8 million pounds ($160,000, 135,000 euros) in damages for destroyed property and as compensation to the interior ministry.
Egyptian courts have sentenced hundreds of Morsi supporters to death since his overthrow, but many have appealed and won new trials.
Morsi and other top figures of his Muslim Brotherhood have also faced trial.
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