Consider the charges $q, q$and $-q$ placed at the vertices of an equilateral triangle of each side $l.$ The sum of forces acting on each charge is 

$$\begin{gathered} net force acting on A,\overrightarrow{ F_A}=\overrightarrow{F_{AB}}+\overrightarrow{F_{AC}} \\ net force acting on B\overrightarrow{, F_B}=\overrightarrow{F_{BA}}+\overrightarrow{F_{BC}} \\ net force acting on C,\overrightarrow{ F_C}=\overrightarrow{F_{CA}}+\overrightarrow{F_{CB}} \end{gathered}$$

As we know all the forces will occur pairs 

$\overrightarrow{ F_{AB}}=-\overrightarrow{F_{BA}}, \overrightarrow{ F_{BC}}=-\overrightarrow{F_{CB}}, \overrightarrow{ F_{CA}}=-\overrightarrow{F_{AC}}$

$\therefore \overrightarrow{ F_A}+\overrightarrow{ F_B}+\overrightarrow{ F_C}=0$