Converting grayscale shades into alpha channel

The simplest solution would be to use a mask, but that would require you to remember to make the mask just before you export.

If you want to go this route, select layer 3, and then ctrl for windows/or command for mac click the layer you want to be the basis for the transparency, and then click on the icon down the bottom of the layers panel with the circle in a square. This will give you the transparency you want, but it's entirely manual and you have to remember to update it when you edit the black layer.

You could create an action that selects the layer you want to use and makes a mask from that, but you'd still have to remember to run the action...

This is what I do:

1. Select the layer with transparency
   
2. Click on Menu item Layer, Layer Mask, From Transparency.
   

The transparency will be converted into a layer mask, that you can manipulate from there.

EDIT: in Photoshop CS5 or later.