Security officials told AP that the three dead was an initial toll. They spoke on condition of anonymity as a formal announcement had yet to be made.
Witness Mohammed al-Nimr told The Associated Press that security forces and ambulances quickly surrounded the mosque. He said that it was a shooting and that worshippers stopped the attacker from detonating a suicide bomb belt.
An activist, citing witnesses, separately told the Reuters earlier on Friday that an explosion was followed by a gun attack on Imam Ridha Mosque in the town of Mahasen in al-Ahsa region in the Eastern Province.
Residents said security forces were exchanging gunfire with five “terrorists”.
No one has yet claimed responsibility for the attack, Al Jazeera reported.