How can I control media volume from a 3-button in-line remote?
You can use a headset controller app from the playstore (e.g. Jays or Philips Headset or HeadsetButton Controller) to use the "middle button" for that. Might take a few tries to find the correct app (if one doesn't work fine, try another -- I did the same and finally found one working perfectly, see my answer here). I'm not sure if they affect the other two buttons at all, or if you even could configure them -- but they allow me (having a headset with a single button) to perfom all the most necessary functions like play/pause, skip, vol+ and vol- with a single button.