I would strongly recommend that you book a site visit before your stage 3. This way you can really see what the job is about and it will really help you answer some of the questions. Plus it shows that you are willing to go that little bit further for the job i.e. you've used your own initiative and done your own research. Not only will it really aid you in the interview it is also a really good opportunity and lots of fun! I went to Heathrow which was just awesome! I got to plug in and listen to all communications for as long as I wanted. I also had a tour of swanwick the day before which was good for asking last minute questions.
As for the medical - you aren't required to do any fitness things other than blowing into a tube to measure your peak flow. Your height and weight are also taken but I wouldnt worry too much about being fit - I'm not and it didnt affect me!