Counting

a1 a2 a3
a4 a5 a6
a7 a8 a9

This is one layer, we have 2 more layers like this just above it (bn and cn)

So, a1. b1, c1 (9 ways)
Similarly for other two dimensions

So, 27 ways

Now, a1, a5, a9 and a3, a5, a7
So, for every layer 2 ways and we have three dimension and every dimension has three layers

So, 2× 9 = 18 more

Now, 4 diagonals of the cube

So, 27 + 18 + 4 = 49 sets