I'll say the same here that I said to one of the HR people at the day of my stage 3. I told him I was writing on PPrune but also said that even if I hadn't been asked not to reveal anything, there would not be any real need.
The only thing I can give you that you don't know are the actual questions, stage 3 is exactly what it says on the box. A group excersice and an interview with questions surronding the key compentencies outlined in the information about stage 3. If you prepare according to what NATS tells you and have the competence they require, you should be able to pass stage 3. If you are not, well then you have another 2 tries before you're out.