Produces 3-methyl-1-(2,4,6-trihydroxyphenyl)butan-1-one (phloroisovalerophenone). 3-methylbutanoyl-CoA + 3 H(+) + 3 malonyl-CoA = 3 CO2 + 4 CoA + phlorisovalerophenone Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.