Need Photoshop “Difference” Plugin

I want to make an image layer that consists of all the parts of a second layer that differs from a third layer.  That is, for each pixel, compare layer 1 and layer 2.  If they differ, put the layer 2 pixel in that position in layer 3.

I haven’t found a way to do this with the image calculation stuff yet, though I have the germ of an idea about using this to create a mask giving those pixels, and then make the layer from that.

Has anybody already done this?

Why, you ask, do I want this?  Because I’m being silly / anal, basically. There are still  a few tools that work most usefully by modifying an image layer. I’d like to be able to make a background copy layer, do some work with those tools, then automatically create a layer containing only the pixels I changed, and delete the background copy.  This makes it much easier to take a second (or fifteenth) pass at some bit of retouching without putting other work at risk, and is pleasingly consistent with the philosophy of lossless editing.

Because I’m using it on individual layers of a Photoshop image, a standalone utility (even if it understood Photoshop files) wouldn’t be very convenient; I need a plugin, or a Photoshop action.

ETA: The germ didn’t work, but another  new idea did pretty well. The key point was using threshold on the differences. I do each channel separately, apply threshold to each, and combine the resulting masks, then make a new layer via copy. The action doesn’t properly clean up after itself yet, and I haven’t tested it in the presence of existing alpha channels or other complexities. And I haven’t applied anything stronger than visual tests to its accuracy. But I’m already using it.

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