2. go to the channels tab
(you should see the three RGB channels If you don't go to Image/Mode and select RGB color)
3. At the bottom of the window, click the "create new channel" button
4. Individually select one of the channels, I picked green
5. Now select the Rectangular marquee tool
6. Outline the whole channel and hit ctrl+C to copy it
7. Now select the new channel (alpha 1) and press ctrl+V to paste the green channel
8. Below there is a texture used for reference, you need to adjust the brightness and contrast according to the texture. If you want some thing to stand out, It should be closer to white. Im pretty sure black is 0% opacity (transparent) and white is 100% opacity, meaning it will stand out more.
From my experience, in a dxt3 alpha white is ambient and black is rough.
Edit: i just realized how terrible my example is, because a metal texture like this should be DXT1 no-alpha.