When you load the exported file into Matlab you will see at least four variables, namely Axis0, Axis1, Axis2 and Snapshot0. The array with data is in the latter but shaped as N x 3, where N is the number of point samples .
In Matlab, you can reshape the Snapshot0 quantity in the following way.
E = reshape(Snapshot0, [length(Axis0)-1, length(Axis1)-1, length(Axis2)-1, 3)
which you can then use as:
where c is an index to the field component and i,j,k are indices to the discretization of space.
V = reshape(Snapshot0, [length(Axis0), length(Axis1), length(Axis2))
where i,j,k are indices to the discretization of space.