Replacing the #3 cable is rather tricky and I didn't manage to find a good description. So having done it, here is what I did:
* Note the number 3 cable is held down by a piece of metal which is folded over to make 3 clips. Google "iPhone 3G Proximity Sensor Cable" to see pictures of the cable and the metal cover with 3 bent metal clips
* use a fine pointed bent metal probe tool to unhook the metal clips.
* ( I used http://www.mendapump.com/MendaCatalog/Tools/Probes/StainlessSteel/35123/ and http://www.mendapump.com/MendaCatalog/Tools/Probes/StainlessSteel/35623/
This is tricky but doable and a lot easier than removing the glass to get to the clips as I saw on one YouTube video.