This is straightforward, really, but . . . .
A at point (0,0) (charge +Q),
B at point (-a, a) (charge -3Q) &
C at point (a,a) (charge +Q)
I have found the magnitude of the force acting on C due to the other charges (((sqrt 2)/4,∏ E0) * Q^2/a^")
but need to find the unit vector which specifies it's direction.
I need to find the displacement vectors for each of these other points B & C.