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 lysosomes. In concert with the BLOC-1 complex, AP-3 is required to target cargos into vesicles assembled at cell bodies for delivery into neurites and nerve terminals (By similarity). The AP-3 complex associates with the BLOC-1 complex. Component of the coat surrounding the cytoplasmic face of coated vesicles located at the Golgi complex. Belongs to the adaptor complexes medium subunit family.