I have a very simple composition, that features some 3D text (classic 3D mode) a single light, and a comp sized layer which I am using as a shadow catcher.
I turn the shadow catcher layer to 3D, then move it behind the text. great, I see shadows. OK, so I don't want the shadow catcher layer to be visible, just the shadows, so I set the layer materials option to 'accept shadows only'.
Well the shadow catcher layer just turns solid white. I still get the shadows on it, but it's white.