Basically there is 1 area and it is all 1 material. It consists of 3 parts/UV maps: Mountain, Soil, and Beach. The UV maps "overlap" each other which means that the faces of transition between the parts are in both/all three of the UV maps.
The layers of the textures have an order: First there is the mountain, then there is the soil which covers a bit of the mountain, then there is the beach which covers some of the mountain and the soil. The soil and beach textures have slightly transparent parts at the transition.
I use the general Color/Normal/Occlusion/Specularity setup for each of the textures. In the end the different parts shall all have their own diffuse and glossy shaders, the mountain f.e. gets a slightly red gloss to it.
Now there is the problem that the faces of the transition seem to be "diffused twice" you can see that they are way brighter: (note that it really is just the faces of the transition not the whole mountain even if it might look a little like this in the screenshot.)
Last but not least the .blend-file(too large for blend exchange):
To sum it up: Is there a way to diffuse it all with the same brightness whilst remaining separate color and normal maps plus separate specularity maps and glossy shaders. Aka "how to set it up correctly".
EDIT: Here are the textures packed in a .zip:
EDIT: The answer of PGmath might work but in my case it doesn't because the textures are "hand painted" and they already have transparent parts at the borders and stuff. Isn't there another way to achieve this? I cannot use the mask method, I'd have to do the whole thing again.