Landscape material controlled with multiple alpha masks

really need some help on this!

My aim is this:
Create a landscape material using Layer blend, where each material is controlled with an alpha mask. So if I have three materials (Grass, Soil and Asphalt) I want each material to pop up on my landscape tile, exactly where the alpha masks are defining it to pop up.
Sofar I only managed to get the Grass where I want it, on a plane ( just for testing), but I guess this would need a totally different setup going forward to be able to use it on a landcape tile.
Anybody have an idea on how to achieve this?