A three-dimensional semiconductor device may include a substrate including wiring and contact regions and a thin film structure on the wiring and contact regions of the substrate. The thin-film structure may include a plurality of alternating wiring layers and inter-layer insulating layers defining a terraced structure in the contact region so that each of the wiring layers includes a contact surface in the contact region that extends beyond others of the wiring layers more distant from the substrate. A plurality of contact structures may extend in a direction perpendicular to a surface of the substrate with each of the contact structures being electrically connected to a contact surface of a respective one of the wiring layers. Related methods are also discussed.