Another 19 people were handed life prison terms and 10 were sentenced to between 10 and 15 years, judicial and security officials said on Thursday.
Egyptian rights lawyer Khaled el-Masry said military prosecutors had accused the defendants of belonging to the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group and plotting attacks against Christian churches and security forces.
The verdict can be appealed.
ISIL claimed responsibility for the three suicide attacks that targeted the churches in December and April.
Two deadly church bombings in Alexandria and Tanta killed more than 45 people in April, months after a bombing near Cairo’s Coptic Cathedral killed at least 25.
Christian sites of worship across Egypt have been repeatedly targeted in attacks claimed by ISIL, prompting the authorities to impose a state of emergency.
Egypt is fighting ISIL-linked fighters waging an rebellion focused in northern Sinai.