You passed the theory, hazard perception test and test of driving ability to get to where you are now. You need to draw on all of that knowledge to help pass the part 3 test. Any questions the pupil might ask about road signs or laws will come from your Part 1 test. You will spot any mistakes your pupil makes while driving by comparing it to your Part 2 drive.
That way you're demonstrating control of the lesson and will have the pupil's complete attention. Also, you will be able to show visuals to help clarify what you are teaching. But use some common sense - don't continually pull the pupil over!
Don't leave your preparation until the last minute. Anything you need to use for teaching should be at hand and well organised. Not being able to find what you want during the part 3 test can lead to panic and will also look unprofessional.
You should be concentrating on your Part 3 Test 100%. You don't want to arrive at the test centre in a rush. Also, if the worst happens and you fail, you won't feel like teaching anyone after the test.
Getting to know the roads near the test centre is very helpful and can help eliminate surprises. PDI's have failed their part 3 test on the road outside the test centre because they didn't Q&A any potential situations that may arise e.g. there might be cars parked along the road outside the test centre!