Answer
We can rank the pairs according to the magnitude of the electric field:
$(top-bottom) ~\gt~ (front-back) ~\gt ~(left-right)$
Work Step by Step
We can find an expression for the magnitude of the electric field in each case:
left-right:
$E = \frac{V}{d} = \frac{V}{3L} = \frac{1}{3}\cdot \frac{V}{L}$
top-bottom:
$E = \frac{V}{d} = \frac{V}{L}$
front-back:
$E = \frac{V}{d} = \frac{V}{2L} = \frac{1}{2}\cdot \frac{V}{L}$
We can rank the pairs according to the magnitude of the electric field:
$(top-bottom) ~\gt~ (front-back) ~\gt ~(left-right)$
