Quote: "why are there always D, N and S files of a DDS file?"
This is so the shader knows how to use these images.
Commonly you will encounter :
_D for Diffuse map. The basic color and detail of a texture.
_I for Illumination map. Tells the shader which parts are glowing or self-emissive.
_N for Normal map. Tells the shader about the relief of the surface.
_S for Specular map. Tells the shader about how matte or shiny the surface is.
Less commonly you will find:
_Cube for Cube map. This is the image used to simulate reflections on a glossy surface.
_R for Reflectivity mask. Tells the shader which parts are glossy and reflective. Determines how strongly the _cube map appears. This will not always be a separate . DDS. On player weapons, for example, this image is stored in the alpha channel of the _S map.
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