Part of the AP-3 complex, an adaptor-related complex which is not clathrin-associated. The complex is associated with the Golgi region as well as more peripheral structures. It facilitates the budding of vesicles from the Golgi membrane and may be directly involved in trafficking to the vacuole (By similarity). Adaptor protein complex 3 (AP-3) is a heterotetramer composed of 2 large adaptins (APL5 and APL6), a medium adaptin (APM3) and a small adaptin (APS3). Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Belongs to the adaptor complexes large subunit family.