Not sure if it is possible(I think so)you should be able to do a multi pass shader using 3 RGBA skins, each pass reads and applies the different skins to the RGB, then the A(alpha) defines where you cn see through the next RGBA skin and it's textures applied etc.. etc.. so with 2 passes you get 6 textures with decent blending, with 3 you get 9, I don't see a reason for 9, but someone else may need 9, and the final pass of course is the shadow map, so my math here is wrong somewhere....you get the idea though.