If you have a trainee driving instructor licence, get as much practice teaching pupils as possible. Some driving schools will help trainee driving instructors gain practice and then give them a job as an ADI when they’ve passed the Part 3 test.
You can take the test again if you fail it at your first or second attempt. If you fail the test a third time, you’ll have to retake and pass the theory test (ADI Part 1) before you can retake the driving ability test and the instructional ability test again. You’ll have to wait two years from when you first passed the ADI Part 1 test before you can take it again.