Flow over a flat plate in 3D
 

I am modeling flow of air over a flat plate, in 3dimensions. Following was my procedure. I created a rectangular domain whose bottom was maintained as wall. Top as symmetry. left and right side as inlet and outlet respectively.

I am confused as to what should be my boundary condition for the front and back faces of the rectangular domain? I have tried symmetry, wall with no-slip condition etc. I do get a converged solution. Even the velocity profile over outlet is quite similar. However, the correct velocity profile as observed in 2D analysis is not obtained in any of the above cases.
pressure contours 2D
https://imageshack.com/i/nr8gr6j
https://imageshack.com/i/nr8gr6j pressure contours 3D
https://imageshack.com/i/nlsflojhttps://imageshack.com/i/nlsfloj
x-velocity plot for 2D on outlet
https://imageshack.com/i/mvpgxbj
https://imageshack.com/i/mvpgxbjx-velocity plot for 3D on outlet
https://imageshack.com/i/nducaezj
x-velocity vectors for 2D
https://imageshack.com/i/n6p7wkkjhttps://imageshack.com/i/n6p7wkkj
x-velocity vectors for 3D
https://imageshack.com/i/nlx1nyj
https://imageshack.com/i/nlx1nyj