I'm confused now. I don't know anything "3 section". All I mean is on your photo where you've written "Water falls at speed down roof, hits top of gable and deflects pushing existing water under the tiles if sufficient flow". The flashing under your yellow arrow looks as if it sits on top of the slates of the main roof, and is under the slates over the window.
You could be right - maybe the flashing should be on top of both sets of slates, not underneath as I was proposing. Also perhaps I should not refer to it as "flashing" but as "lead valley" as someone else described it.