Three files per texture.Great finding, but why 3 files per helmet:
*_d stands for diffuse map:
The diffuse map is your basic color texture, here you will define the color information for your materials.
*_s stands for specular map:
The specular map is used to define both the color and strength of the specular reflection highlights.
*_n stands for normal map:
The normal map provides per pixel lighting/shading information.