Two similar small spheres having +q and -q charge are kept at a certain distance. F force acts between the two. If in the middle of two spheres, another similar small sphere having +q charge is kept, then it will experience a force in magnitude and direction as

+Q and -Q charges on two similar spheres are kept at a certain distance. F force acts between the two. if at the middle of two spheres, another similar spheres having +Q charge is kept, then it experiences a forces in magnitude and direction as 8F towards -Q charge
$$\begin{gathered} \mathrm{F}={\mathrm{F}}_{\mathrm{AB}}=\frac{1}{4\pi {\epsilon }_0}\cdot \left(\frac{-{\mathrm{Q}}^2}{{\mathrm{r}}^2}\right) \\ {\mathrm{F}}^{\mathrm{\prime }}={\mathrm{F}}_{\mathrm{AO}}+{\mathrm{F}}_{\mathrm{OB}}=\frac{1}{4\pi c0}\cdot \left(\frac{4{\mathrm{Q}}^2}{r^2}\right)+\frac{1}{4\pi \mathrm{c}}\cdot \left(\frac{-4{\mathrm{Q}}^2}{r^2}\right)=4\mathrm{F}+4\mathrm{F}=8\mathrm{F} \end{gathered}$$

The sphere experiences 8F force towards (−Q) charge.