Is an electrical box fill classified by wires, cables or conductors?
Each current carrying conductor  (hot, neutral) that enters the box, counts as one (1) box fill unit. All the grounding conductors together, count as one (1) box fill unit.  Pigtails do not count. The device counts as two (2) box fill units.

Total all the box fill units, and multiply by the multiplier listed for the conductor size. 

Three 12/2 with ground is seven (7) box fill units, plus two (2) box fill units for the device.  That's 20.25 cu.in.