Gawd my Stage 3 was a long time ago but from what I remember the advice on the careers website was pretty accurate. Just be prepared for questions on the topics listed and have answers ready for them. If you have the required qualities you should get through. Simples!
I've no idea about selection dates but from the rumours I've heard there's over a year to wait before you start on a course after passing Stage 3. Due to the surplus of trainees at units NATS have scaled back the course sizes. The next one to start is probably no more than 10 TATC's.
Worth the wait though!