Radio dzXL reported that the grenade was thrown at the house of Maguindanao National High School principal Maimona Ampatuan in Shariff Aguak town.
Those hurt were identified as Enrico Ampatuan 8; Norhana Abdulrahman, 13; and Monera Camsa, 30.
Witnesses said two men aboard a motorcycle threw the grenade at the house.
The explosion also damaged three vehicles.
Investigation showed the principal was the target of the attack.
The principal had received several threats allegedly from extortionists, the report said
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