I am attending my stage 3 in 3 weeks

. How on earth do they fill up the whole day.
What are the group tests?
How long is the interview? -is it tense?
Please throw some advice, I know its not exactly fair to be getting such a heads up at such a final stage... but there is not much stage 3 info around.
does any1 know if they DO have availablilty on the March and June intake dates... reason I ask is because I would like september- so would they see me as being difficult if i requested THAT!?
I went to a nats and a non-nats tower, read a whole lot too- So I'm not looking for any shortcuts. I just wanna have a better picture of the actual day. thanks in advance