Well, I use mtl_terrainmulti3 in my game and it works fine. In the first skin I've got the black, red and green skin. Then in skins 2,3 and 4 the three different textures. The first texture goes over all the black pieces, the second over the red areas and the third over the green areas.
I have however not been able to get the blue to work - just shows up as a LIGHT spot instead of as a shadow. Have you set up your skins like this?