Three capacitors each of capacitance C and of breakdown voltage V...

In series arrangement charge on each plate of each capacitor has same magnitude. The potential difference is distributed inversely in the ratio of capacitors ie, $\mathrm{V}=\mathrm{V}$_1+\mathrm{V}_2+\mathrm{V}_3 \ldots Here, $V=3 V$ The equivalent capacitance $\mathrm{C}_s$ is given by
\begin{aligned} & \frac{1}{C_s}=\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}+\ldots \\ & C_s=\frac{\mathrm{C}}{3} \end{aligned}