Rolfeo:
The stage 3 interview is a competency based (or structured) interview. I'd highly recommend doing some research into this type of interview and how it works (try googling it, or visiting the careers sections of your local library) as it is probably quite different to any kind of interview you may have done before.
The competencies you are being tested for are all listed on the pdf you mentioned.
...and yes you can fail at the interview, it is just as important a part of the selection process as everything else.
Jenga-Jen:
As you suggest, the medical department are probably just checking your glasses prescription (its unlikely there would be anything else that would show up on your initial application that would require investigation at this stage that you wouldn't already be aware off). They are usually pretty quick but I wouldn't fret about it, the selection process can be pretty long term (I started 6 months ago and still don't have a start date just yet) so one or two days isn't really going to make a major difference in the long run.