electrostats

work done in bringing the charge from infinity to this point...

we use superposition...

assume only charge q2 first so the potential will be kq1q2/a

then only charge q3 ... potential will be kq1q3/a

I hope this solves it for u :)

hmm.. that is true..

but the problem is that u have not asked the potential energy of the system but of the single charge!

I hope this helps?

Potential energy is the energy required to bring a charge from infinity to that point ((it is already taken that there are many more charges in the system.. which cause an electric field))