(Adds details, increases toll to 3)

GENERAL SANTOS, Philippines, Jan 30 (Reuters) - A bomb blast at the gate of a tuna canning factory in the southern Philippines on Wednesday killed at least three people and wounded 21, police said.

Robert Po, the police chief in General Santos city, said the explosive was a home-made bomb and went off just after dusk when workers were ending a shift.

The attack was in General Santos City, a port on the southern tip of the peninsula that is the country's biggest centre for tuna processing. Six of the country's eight tuna canneries are located in the city.

No one claimed responsibility for the blast.

"We are still investigating whether this is an extortion attempt or labour-related," Po said. He said he was also not ruling out an attack by Muslim radicals.

General Santos is Catholic majority, but is bordered by Muslim-dominated areas. Several Muslim groups are fighting in the region for independence, but the largest rebel group, the Moro Islamic Liberation Front (MILF), is in peace talks with the government of the Catholic nation. (Reporting by Manny Mogato; Editing by Sanjeev Miglani)

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