Selecting the Texture in Texture Paint Mode I added a shader and use 3 image textures which are combined with mix RGB. How to choose the texture I want to paint in the 3d viewport when doing texture painting?
You are needing the Texture Slots tab in the Properties panel in the tool properties. Select the appropriate slot to determine where your paint will go. Here I am using two image textures set to box mapping and one black generated image as a UV mapped stencil between them.

I used an RGB Color Mix node to combine the two textures with Multiply mix mode, so that the rust adds to the original texture instead of replacing it.