Formation of a macromolecular complex between proteins of the AP-3 adaptor complex and proteins and/or lipoproteins that are going to be transported by a clathrin-coated vesicle. In some cases, the AP-3 complex is a heterotetrameric AP-type membrane coat adaptor complex that, in some organisms, links clathrin to the membrane surface of a vesicle.
cargo loading into clathrin-coated vesicle, AP-3-mediated