Potential energy of an assembly of three charges

Work done in bringing second charge to the first is 0.9 J. A third charge brought in the vicinity of the first and second charge is the sum of work done in bringing the third charge near the second and the first charge , let it be 1.8 J. The total work done in assemblying these three charges is 2.7 J . The work done is stored as potential energy. Therefore the potential energy of an assembly of three charges is 0.27 J ( the sum of bringing second to the first and the third to the first and second)