Is that possible?
Baking from an object that has 3 textures each referencing a different uv layout into an object with only one texture (the baked one) that combines all 3 onto 1.
I am putting decals on the car (each a separate image) and want to bake them all into one.
Each time I try I am unsuccessful 
Bake into separate images which you can then consolidate into a single image with the Texture Paint Plus addon https://sites.google.com/site/bartiuscrouch/scripts/texture_paint_plus
Absolutely possible. Make sure that the rendered image shows the right textures, that the UV layer that you want to use for the combined texture is active and has an image to bake to, and then just hit the bake button (with ‘Textures’ mode selected).
Here are three faces of a cube UV unwrapped over one image, and the other three unwrapped over another one (using a different UV layer, even though that isn’t really needed):
And here is the bake to a lightmap pack on a third UV layer:
Best wishes,
Matthew
