I obviously don't know what will have changed in the new Stage 3, but I'd be surprised if they didn't ask you anything about ATC and the technical aspects of it. I'd still expect (and prepare) for technical questions - if you go and visit a NATS site, I would still think you'd be asked about it (and what you saw there) as it's a good test of enthusiasm, interest and understanding.
Anyone who had the old Stage 3 were presented with scenarios (not all ATC based but some of them were) to see how we'd apply knowledge we either already knew or was supplied to see how we think, and I'd be very surprised if the interviewers don't think that's necessary any longer.
Read the motivation paper for stage 1 again, go visit sites and read up on anything else you can find (Graham Duke's book is good, especially if you guys haven't been sent the old Stage 2 motivation document). Prepare for the HR stuff too - it's a large chunk of the interview and just as important as anything else... make sure you have good examples of your personal qualities and are prepared to speak freely and honestly about your past.
If the technical revision doesn't help you for the interview, it will help once you get to college - we'll have enough to learn when we get there!
Good luck for all those going for the final interview soon!