I have my stage 3 on 12th july. the kind of stuff i have been looking at is the aircraft that each partner airline tends to fly, and the usual routes for that aircraft. I think researching such stuff as MTOW is a bit much, but as has been said its better to be over prepared. I think we will be required to have a basic knowledge about why planes fly (ie the basic principles) and have an understanding about how airlines operate. believe it or not, i have been using such programs as airline and airport to understand the airline side of thing, whilst seconds from disaster and air crash investigation to learn about the technical side (plus it gives me an excuse to be an anorak!!).
Apparently from what I have heard though, it its the questions about yourself that are the more taxing to answer. So dont forget to think about why you actually want to become a pilot and try and think of good experience of things from your past (such as team working, voluntary etc).
For the group activities, the key is to take an active part but not be overbearing. Make sure the assessors can see that you are getting involved.
Having said all that, it would now be sods law for me to fall down. if anyone else has their stage 3 on 12th, gimme a buzz.