could any one tell me how to give inlet velocity parabolic profile for a backward facing step flow in 3D. please

It depends on what type of backwards-facing step flow you wish to investigate. Shear layers are particularly sensitive to their initial conditions in experiments, and similar tendencies have been shown in simulations. If the separating boundary layer is laminar in nature, one can use a blasius-type profile, with small f.s.t. disturbances added to it.

For a turbulent separating boundary layer, it is much better to use time-dependent inflow data obtained from precursor boundary layer simulations, as these will give you the large-scale temporal variations required for a turbulent-type inflow condition. For information on this, see the papers by Tenaud et al. (Computers & Fluids 2005), and Li et al. (Phys. Fluids, 2000) for more information.

could you please tell me the title of the paper by Tenaud et al and Li et al.

Large Eddy Simulations of a spatially developing incompressible 3D mixing layer using the vâ€"ω formulation - Tenaud et al..

Inflow conditions for large eddy simulations of mixing layers - Li et al..

In answer to your question, you might look at Eq. 3-48 (page 120) in the book Viscous Fluid Flow, 2nd ed by Frank M White. Fully-developed flow in a rectangular duct is one of the problems which can be solved "exactly." The equation should be given in many other textbooks, as well.

Unfortunately, the velocity in 3D does not have the simple parabolic form as in 2D. The solution is presented as an infinite series, which I will not try to write out here.