Why don’t you import the “splat maps” as raw files into the terrain UI instead? It’s a waste of 2D sheets to have them inside of the material since you’ll be very limited already with the amount of textures you can use before you reach the limit in the shader.
Having them inside of the material must have been outdated years ago, I set up my material back in UDK without that at least two years ago and it’s all in the official documentation. Just read : .unrealengine.com/latest/INT/Engine/Landscape/Materials/index.html