Combining separate, post-processed Cameras

I have three cameras, each seeing only a single layer, and want to add separate post-processing effects to each of them and combine them afterwards.
It is important to note, that the combining has to be done afterwards, as I do not want the shader of camera 1 to recolor everything below it.

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For the record I’ve been trying with Graphics.Blit, but cannot figure out how to preserve the transparency/stencil buffer between “blits”.

Okay - some months later here I am.
I ended up using the depth texture of the camera to apply different effects depending on the depth of the objects.

I still do not know if this is possible.