If \( z_1, z_2, z_3 \in \mathbb{C} \) are the vertices of an equilateral triangle, whose centroid is \( z_0 \) then \( \displaystyle\sum_{k=1}^{3}\left(z_k - z_0\right)^2 \) is equal to:
\( 1 \)
\( 0 \)
\( i \)
\( -i \)