Type III polyketide synthase that catalyzes the synthesis of phloroglucinol from three molecules of malonyl-CoA (By similarity). Required for the synthesis of the polyketide antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) (PubMed:10322017). 3 H(+) + 3 malonyl-CoA = 1,3,5-trihydroxybenzene + 3 CO2 + 3 CoA Antibiotic biosynthesis. Mutant cannot produce monoacetylphloroglucinol (MAPG) or 2,4-DAPG. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.