Compositing: When using Alpha Over node, only one render layer appears! I'm new to compositing, so I'm unsure how simple this problem may be to solve; apologies in advance. Anyway, here we go.

My scene was too large to render, so I separated it into 3 different Render Layers- 



The three layers seem to be rendering correctly, lighting and all, as the render progresses; I can see each individual layer as it should be, and have my Alpha Over nodes set up accordingly-



However, the final render saves to my computer as such-



Some final notes; I have transparency selected in the render panel, the render also doesn't work if I remove the "File Output" node, and I have it set to GPU render (though it's the same result even if I change to CPU render).

What am I doing wrong!?
You render layers seem fine, What I think is missing is compositing checked in the Post Processing Panel in the Render tab.

Edit you need to have all 3 layers active in order to work, in the scene part of the Layers Panel, only the layer  3 is active. Shif select the other two layers to enable them.