mike,
there is little you can do to prepare for mental arithmatic and even less for the pilapt; just make sure that you are thinking straight and on top of your game - don't worry about digging out your old maths notes or playing flight sim until the early hours.
The discussion could be on any subject(literally!) and is orientated on your ability to participate without being overbearing.
Stage 3 requires a little more preparation in the form of background research (into the company, and wings in particular)and answers for obvious questions such as why you want to do it - sounds simple but try explaining it in an interview!
all the best