The supreme criminal court also stripped eight of the defendants of their Bahraini citizenship over the March 3 bombing in a Shiite village, the deadliest attack since security forces crushed Shiite-led protests in March 2011.
Among the three policemen killed in Daih village was one officer from the United Arab Emirates.
First Lieutenant Tariq al-Shehi was the first foreign officer killed since Saudi-led troops and police deployed to the kingdom to support its crackdown on the Arab Spring-inspired protests.
The court heard that the defendants detonated the bomb after mobilising protesters to join a funeral in Daih village to lure in police.
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Prosecutors said defendants had planted three bombs in all but only one was detonated.
They charged that two of the defendants were among the founders of the little-known Al-Ashtar Brigades, a group that has reportedly claimed responsibility for several attacks in the Shiite-majority kingdom, including a July 2013 bombing outside a Sunni mosque.
Tiny but strategic Bahrain, which is home to the US Navy's Fifth Fleet, has been rocked by unrest since the crushing of the 2011 protests for parliamentary government and a constitutional monarchy.
At least 89 people have been killed in clashes with security forces, while hundreds have been arrested and put on trial, human rights groups say.