For dolfinx: see for instance dolfinx/python/demo/demo_gmsh.py at v0.8.0 · FEniCS/dolfinx · GitHub to verify that the answer to 1 and 2 is yes for dolfinx. Further lower level details are shown in dolfinx/python/test/unit/mesh/test_higher_order_mesh.py at main · FEniCS/dolfinx · GitHub
The answer to 3 is probably yes too, except that you’ll need to write your own reader.
For dolfin: see for instance Is it possible to program (from scratch) a 2nd-order tetrahedron in FeniCs Legacy? . The short answer is that we’d suggest moving on to dolfinx anyways.