The electrostatic potential energy of a system is the work done in assembling the charges from infinity. For two charges: U = $\frac{kq1q2}{r12}$. For three charges: U = $\frac{kq1q2}{r12}$ + kq1q3/r13 + kq2q3/r23 (3 terms). For N charges: N$\frac{N-1}{2}$ pair terms. The energy can be positive (repulsive configuration, work done against repulsion) or negative (attractive, energy released). Self-energy (the energy to assemble a single charge from its constituent parts) is usually excluded in JEE problems. The total energy equals the negative of the work done by the electric field during assembly, or equivalently, the work done by external forces. To find binding energy of a charge in the system, calculate the potential at its position due to all other charges and multiply by its charge.