Three rigid rods are connected to create an equilateral triangle...

Since the complete system is stationary in an inertial frame, the acceleration of each charged particle is zero.

According to Newton's first law of motion, the net (resultant) force acting on any stationary particle must be zero.

The resultant electrostatic force of repulsion on the particle at $A$ (which is $3.6\sqrt{3} \text{ N}$) is exactly balanced by the mechanical reaction forces exerted by the rigid rods to which it is fastened.

Therefore, the magnitude of the total resultant force experienced by the charged particle at $A$ is zero.