The Coulomb force ($\vec{F}_{\text{Coulomb}}$) exerted by one charge ($q$) on the other ($q$) is a repulsive force.
If we consider the force exerted on the lower charge ($q$ at $x_3 = -d$) by the upper charge ($q$ at $x_3 = d$), the vector force is: $\vec{F}_{\text{Coulomb}} = \frac{1}{4\pi\epsilon_0} \frac{q_1 q_2}{r^2} \hat{r}$, With $q_1 = q_2 = q$, distance $r = 2d$, and the unit vector pointing from the source (upper charge) to the test charge (lower charge) being $-\vec{e}3$ (downward): $\vec{F}{\text{Coulomb}} = -\frac{q^2}{4\pi\epsilon_0 (2d)^2} \vec{e}_3$