Three identical point charges, each with charge \(+q\), are initially fixed at the vertices of an equilateral triangle with side length \(d\). The distance from any vertex to the center of the triangle is \(\dfrac{d}{\sqrt{3}}\). An external agent slowly moves one of the charges from its vertex to the center of the triangle while the other two charges remain fixed at their original positions. How much work is done by the external agent to move the charge to the center?