Your posts are a bit confusing Bowdown. To test the gauge, did you remove the gauge and carry it around to the battery and hook up 2 wires- one to each of the prongs on the back of the gauge, and then touch the two wires to the battery terminals, one to the + and one to the - side of the battery? If so, is this where you are getting a 3 volts reading? If you didn't check the gauge this way, then do so. See if the gauge reads 11-12 volts. If the gauge needle goes backward, you have the wires backwards. Switch them to the opposite battery posts and get your reading.
To answer one of your questions- yes, there are 3 metal clips on the back of the gauge. but, as Beastie said, only 2 are used.