Answer
$U_3=2\times10^{-2}J$
Work Step by Step
We know that
$U=\frac{1}{2}CV^2$
For capacitor 3, this simplifies to:
$U_3=\frac{1}{2}C_3V_3^2$
We substitute the known values to obtain:
$U_3=\frac{1}{2}(4\times10^{-6})(100^2)$
$U_3=0.02J$
$U_3=2\times10^{-2}J$