Clone texturing: how to indicate to the brush to use a particular MAP to paint?

I can’t understand texture painting cloning textures, it seems.

I’ve watched some videos but it isn’t sufficient because to me it doesn’t work as expected.
I use 2.77 and Cycles.

I have a mesh of short pants and I have created 3 textures (Front, Side, Back) for it.
In Blender, I have created 3 UV maps + 1 called “main” that contains the UVgrid as texture and all are in the same Material. The 3 UV maps are made with only a group of vertexes, not all of them, so for the frontal texture I had selected only the frontal vertexes ecc.
I tought it was all ok… Until I had to merge my projections in the main unwrapping.
If it matters, in Nodes I have 4 Texture nodes, one for each texture, and only the Main one (the uv grid) is connected to the Diffuse node.
Well, when I use Texture Paint and the Clone brush, I have 4 slots, one for each texture. I try to select the Front slot in the brush; the “main” UVmap in the list of the UVmaps; and the uvgrid in the UV map visual section in order to paint on it as result.

It should work? It doesn’t work as expected. If I start to paint that Front slot on the “main” uvgrid, it paints like if I didn’t mapped the frontal group of vertexes. It clones from the native image, ignoring the Front mapping on it, so the final painting results equal like if I simply apply the native frontal texture under the final main unwrapping.
I have to admit that I’m confused about how all of these sections communicate together if my attempts are wrong.
I was meaning the slot as the source image to paint on the grid, and the UVmap as the coordinates that the Clone Brush uses to paint it. So the problem is that I don’t know how to indicate to the brush:“Use THIS FRONTAL MAPPING to paint this frontal texture on the final main map” :confused: