Terzo is a comune in the Province of Alessandria in the Italian region Piedmont, located about 70 kilometres southeast of Turin and about …Terzo is a comune in the Province of Alessandria in the Italian region Piedmont, located about 70 kilometres southeast of Turin and about 30 kilometres southwest of Alessandria. It was a fortress of the bishop of Acqui, of which only a late medieval tower remains. Terzo borders the following municipalities: Acqui Terme, Bistagno, Melazzo, and Montabone.