If you haven't read those, what have you read? With just 3 weeks left, you better hop on it!
I say go ahead and take 1 test to see where you stand. After that, work through Destroyer as quickly as possible. You're probably going to want to go through this book 2 or 3 times.
Since you have 3 weeks left, take 2 tests a week. It'll give you a way to guage your progress.