What would be the voltage across C3?

The capacitors C1 and C2 are connected in parallel across points A and B. Thus, the effective capacitance between A and B is CAB = C1 + C2. Using Kirchhoff's loop law in the second figure, → −V + q/C3 + q/(C1 + C2) = 0 → q = V C3 (C1 + C2) / (C1 + C2 + C3) ∴ Voltage across C3 is V3 = q/C3 = (C1 + C2)V / (C1 + C2 + C3)