A parallel beam of light strikes a piece of transparent glass having cross section as shown in the figure below. Correct shape of the emergent wavefront will be (figures are schematic and not drawn to scale)

As we can see that the thickness of the transparent glass is minimum for the rays which are in the middle portion. So these will have the minimum optical path and the rays on the extreme ends will have to cover the maximum optical path length. Hence, the wavefront of wave 2 will be ahead of waves 1 and 3.