Buy the C&G 2391 exam success book. Well worth the ÂŁ15 or so quid. It is a massive help for the written exam, with lots of tips for saving time, especially if like me you are not the best artist that ever graced the planet, and can sometimes ramble on a bit when trying to describe in detail something. This book shows you how to cut the crap and save time.
15 weeks is a good period to get to grips with GN3, learn a bit at a time, and you will have no big trouble on this course or the exams.
The biggest cause of failure is lack of revision time, so if you have 15 weeks, you stand more chance than someone who is going to take it the week after.
Cheers............Howard