Capacitors in series:
When three capacitors are connected in series. If one electron moves from the negative terminal of the cell to the right hand plate of C3, and one electron moves from the left hand plate of C1 to the positive terminal of the cell, one electron will move between C1 and C2 and between C2 and C3. Thus, the total charge moved is one electron and the charge on each capacitor is one electron. Thus:
QTOTAL = Q1 = Q2 = Q3
but V = V1 + V2 + V3 (Kirchoff's second law)
also V = Q/C
Therefore Q/CTOTAL = Q/C1 + Q/C2 + Q/C3
Therefore, the three single capacitors may be replaced by a single capacitor whose capacitance (C) is given by the above equation, provided its safe working voltage is of a sufficiently high value to withstand the applied voltage.