Work Step by Step
As stated, it takes work W to bring Q1 and Q2 together until they are a distance d apart. Bringing in a third charge means it is repelled by each of the other charges. It takes an amount of work W to bring Q3 a distance d away from Q1, and an amount of work W to bring Q3 a distance d away from Q2. The total work to assemble 3 charges is 3W.