Catalyzes the carboxylation of beta-amyrin at the C-28 position to form oleanolate (PubMed:28194155). Catalyzes the carboxylation of alpha-amyrin at the C-28 position to form ursolate (PubMed:28194155). beta-amyrin + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = 4 H(+) + 4 H2O + oleanolate + 3 oxidized [NADPH--hemoprotein reductase] Belongs to the cytochrome P450 family.