Could you be more specific when you say that the pressure does not seem to be correct?
As a 2D and 3D implementation of the Navier-Stokes equation in dolfin only differs when it gets to boundary conditions, you should verify your solver with a 2D mesh.
There are many benchmarks that you can use to verify your implementation, see for instance:
the oasis paper, which used the 2D Taylor-Green problem to verify their solver, which can be found at the Oasis github page. You can also see the multimesh paper on Navier Stokes splitting schemes for fenics, the code is available at Zenodo. In this paper, both the Taylor-Green benchmark, as well as the DFG 2D-3 benchmark was used to verify the implementation. Note that there also exists 3D benchmarks, see this link.