Height and Slope blend with the ability to paint over the Landscape?

I’ve Just created a simple landscape material composed of 3 layers (dirt, rock and grass) with a simple texture blending based on slope and distance.

The effect when I apply this material to my landscape is quite good but I’m unable to paint on it using the brushes since no layer is recognised after assigning the landscape material to the landscape.

To make A Layered material I’ve used the MATLayerBlend_standard Blueprint, how can I modify my project to also have the 3 layers available as “paintable”?

attached you can see the landscape material overall scheme and a sample of one layer material function used (the material function is always the same for each layer)