The complex numbers $z_1,z_2,z_3$ are vertices of an equilateral triangle with circumcentre $z_0$.The value of $z_1^2+z_2^2+z_3^2$ equals

Question

$\begin{array}{1 1}(A)\;z_0^2&(B)\;3z_0^2\\(C)\;\large\frac{9}{2}\normalsize z_0^2&(D)\;\text{None of these}\end{array}$