I forget the exact reason why, I think its because electrical potential energy (EPE) is a state function (path independent) so you can calculate the electrical potential energies in between each of the particles, and then sum them up to get U of the system, which is the same as EPE of the system since I'm assuming all 3 particles are at rest.
k = 9 x 10^9 (I don't recall the units off the top of my head, but you can easily do a dimensional analysis to figure it out; its also equal to [4*pi*epsilon]^-1)
1e = 1.6 x 10^-19 C
EPE = kQ1Q2/d
EPE1 = EPE between 1e and 2e, d is 5 x 10^-3 m
EPE2 = EPE between 1e and 3e, d is 1 x 10^-2 m
EPE3 = EPE between 2e and 3e, d is 5 x 10^-3 m
EPE1 + EPE2 + EPE3 should get 4.38 x 10^-25 J.
Let me know if I'm wrong, or if I'm right and my reasoning happens to still be wrong.
Edit: Just did the calculation, I get 4.3776 x 10^-25 J. So it's correct.
