Suppose you have six squares of stained glass, all of different colors, and you would like to make a rectangular stained glass window in the shape of a 2 × 3 grid.

How many different ways can you do this, taking symmetry into account? (Note that any pattern may be rotated 180°, flipped vertically, or flipped horizontally. You should count all the possible resulting patterns as the same window.)

I've tried 6! / 3 but this is not correct. Any tips?