Three-dimensional semiconductor devices are provided. The three-dimensional semiconductor device includes a substrate, a buffer layer on the substrate. The buffer layer includes a material having an etching selectivity relative to that of the substrate. A multi-layer stack including alternating insulation patterns and conductive patterns is provided on the buffer layer opposite the substrate. One or more active patterns respectively extend through the alternating insulation patterns and conductive patterns of the multi-layer stack and into the buffer layer. Related fabrication methods are also discussed.