In general when there is a system of N charges then to calculate the electric potential energy of the system, we need to sum the energies of each pair of charges formed (not repeated)
Q = 3.0 nC
q = -2.0 nC
Let r1 = 3.0 cm
r2 = 4.0 cm
then, r3 = 5.0 cm
Now the potential energy of the system would be:
£ bo .
U = 9*10^{9}*3.0*10^{-9}* \frac{10^{-9}}{10^{-2}}*(\frac{-2}{3.0} + \frac{-2}{4.0} + \frac{3}{5.0})
U = -1.53 *10^{-6} \ J is the required answer