Thanks a lot for your reply. Actually, I don't know that the inlet boundary condition is applicable only to faces (2D) only and not cells (3D).
For some reason, I cannot draw 2D surface mesh in my meshing software (gmsh), because of this I am asking how to add it in ANSYS after inserting the mesh. When I insert the mesh I can see only (default faces and interior ) in the boundary conditions tab, I don't know where I should define the water.
I am attaching screenshots, the water should enter from Inlet 1, and then will be distributed in the inner area and enter also from Inlet 2 and Inlet 3.
you mentioned I should do (Under Boundary-> Zones -> Separate, a window will open. What you can do is create Local Regions over the desired faces and then Separate the walls. Then using Boundary->Manage you can give these faces a new/more appropriate name.) I don't see these options when I open Fluent.
would you declare it a little for me, according to my case?