A charge ${\mathrm{Q}}_1$ exerts some force on a second charge ${\mathrm{Q}}_2$. 1f a 3rd charge ${\mathrm{Q}}_3$ is brought near, the force of ${\mathrm{Q}}_1$ exerted on ${\mathrm{Q}}_2$ : 

will decrease

will increase if ${\mathrm{Q}}_3$ is of the same sign as ${\mathrm{Q}}_1$ and will decrease if ${\mathrm{Q}}_3$ is of opposite sign 

will increase

will remain unchanged