What would be the voltage across C_3 ?

Capacitors $C_1 and C_2$ are in parallel, their equivalent capacitance is in series with capacitor $C_3$. Hence, the equivalent capacitance of the given circuit is C_{e q}=$\frac{\left(C_1+C_2\right) C_3}{C_1+C_2+C_3}$Charge on capacitor $C_3$ is $$\begin{aligned}Q & =V C_{\mathrm{eq}} \\ & =\frac{V\left(C_1+C_2\right) C_3}{C_1+C_2+C_3} \end{aligned}$$Voltage across $C_3$ is =$\frac{Q}{C_3}=\frac{V\left(C_1+C_2\right)}{C_1+C_2+C_3}$