It's probably 3 separate grayscale textures combined into an RGB image file so red, green and blue channels are separate textures:
The blue one looks like it might be ambient occlusion, but it's hard to tell. So the A in SGA might be for ambient occlusion, but why A why not AO?.. Well, we should ask the person who created this. Anyway, red looks like it might be specular and green seems like glossiness(inverse of roughness).
To use them, just split the channels using Separate Color node and use the outputs of it as any other textures whatever way you want. So it might look something like this, if you want to use Principled BSDF:
It's worth noting that this is evidently not a very smart way to name textures if they are meant to be distributed because it causes precisely this kind of confusion. So if you ever find yourself in a situation where you have to create assets for other people to use, just remember this and give the files proper descriptive names.