Which is the bare wire in a 12-3 gauge cable?
The ground wire will not have any extra insulation, though it is usally wrapped in paper to seperate it from the other conductors.

12-3 NM (aka "Romex") cable will have a bare grounding wire, white neutral (grounded) wire, black hot (ungrounded) wire, and red hot (ungrounded) wire.

If you only see white, black and bare wire then it is actually 12-2 (the second number is the number of conductors, the ground wire is not included in this).

12-3 NM Cable