Marciano Paynor, director-general of the Asean organising committee, said on Thursday; "It is precisely this act and other acts of senseless violence and terrorism that the leaders will be addressing when they meet here in Cebu."
The first and deadliest blast killed six people in General Santos city and wounded 33, police said.
The second explosion injured two more in Kidapawan City while the third, from an 81mm mortar, rocked Cotabato city, killing one person and wounding five.
Oscar Calderon, the Philippine police chief, said the terror groups "were trying to send a message".
"They wanted to embarrass the government because of the meetings in Cebu."
Police said two groups, Abu Sayyaf and Jemaah Islamiyah, may have had a hand in one of the incidents.
Chief Superintendent German Doria, the regional police director, said the mortar shells used in the Cotabato blast were the signature of the two groups.
Both groups have been involved in previous bombings in the Philippines.