1. A three-dimensional (3D) rendering method, using one or more graphics processors and a memory, comprising: determining a vertex for a first shading from among vertices of a 3D model based on characteristic information of the 3D model; performing the first shading on the determined vertex; allocating, to the vertices, flag values indicating whether the first shading is performed for the vertices; interpolating the flag values; determining reference information indicating whether the first shading is applied for all pixels of an image frame based on the interpolated flag values; determining a pixel area for a second shading based on the reference information; performing the second shading on the determined pixel area; and generating a rendered image based on the first shading and the second shading.