Vice Mayor Mojahiid Andi of Dinas town was killed along with police bodyguard PO1 Edgar Somanaban and civilian aide Estelita Ponse in the ambush by about 10 gunmen in Barangay Sambulawan, said Senior Superintendent Karib Muammil, Zamboanga del Sur police director.
Two others, Zaldy Campomanes and Sheila Domingo, were wounded in the attack.
The gunmen blocked the road with fallen electrical posts and a truck to stop a convoy of eight vehicles led by Dinas Mayor Wilfredo Asoy as they were returning from a meeting of provincial officials.
Asoy, who was believed to be the main target of the ambush, escaped unharmed, Muammil said.
Local officials said the attack was believed linked to a continuing feud between the family of political leader Abdul Basit Maulana and a rival clan, the Laos.
Both the mayor and vice mayor were allies of Maulana.
Last month, a defeated mayoral candidate, Satok Lao, was shot dead just outside his home in Dinas town.
Feuds between politically influential families in Mindanao often spiral into violence between their armed followers. — Roel Pareño, Cecille Suerte Felipe and AFP