Good luck on the test. VSP gave you good advice on the reading list. I'd also suggest the previous (1996, as I recall) Alpine Manual, along with the Level 3 Study Guide that accompanied it, and Ron LeMaster's The Skier's Edge. It's likely that someone will have these there for you to borrow but, of course, if you aren't familiar with them, they may not help much. That's really the reason our Written Test is open book in the RM Division--it's nice to know the answer, but it's almost as good just to know where to find it.
Remember though that the Level 3 test, in particular, is designed to make you think and connect the dots a bit. It's meant to test a "Level of Cognition" (Bloom's Taxonomy of the Cognitive Domain: Knowledge-Comprehension-Application-Analysis-Synthesis-Evaluation) well beyond the "Knowledge" level. Creative and technically accurate teaching--the Level 3 standard--does not come from a manual, or even a whole library. There are movement analysis questions, for example, that will challenge your cause-and-effect analysis ability.
It's all multiple choice, so many questions do come right out of the suggested reading materials, as do their answers. But do expect a few that are based on your general knowledge and your ability to synthesize new information from your understanding of the topics covered in the reading material.
For what it's worth, no one is expected to get all the questions right, and it would be a remarkable accomplishment to do so. Teaching skiing effectively requires a working knowledge of many technical fields--physics, mechanics, biomechanics, geometry, anatomy, physiology, nutrition, psychology, exercise and practice theory, communication, and education, not to mention things like history, geography, foreign languages, and so on that can certainly also help facilitate a superior lesson. And then there's the specific understanding of skiing itself, equipment design and setup, tuning and waxing (hint), weather, snow conditions, ski design, and, of course, the ability to make a decent turn!
No one is an expert in all these fields, but there are a few questions from most of them intended to give those who have superior knowledge in some specialty field an opportunity to demonstrate their excellence there, even if they might be lacking in another area. While some people have concerns about this, I think it's not only fair, but it accurately reflects the realities of the job of teaching skiing. Each of us has a unique set of strengths we learn to exploit and weaknesses that we learn to compensate for. And it all works out!
There are also questions meant to challenge a few popular myths and misconceptions. How do flexion-extension ("up and down") movements affect pressure on skis, for example? (Hint: it has nothing to do with the direction you're moving--up or down. Bone up on your basic physics if necessary!) And remember that, if turns are seamlessly linked, they must end and begin in the same position/attitude. That is, regardless of your beliefs about flexing and extending and the optimal timing thereof, for example, the position (flexed or extended) that starts a turn MUST be the same as the position that ends the previous turn, because the end of one turn IS the start of the next if they are to be linked.
All told, in a perfect world, the written test would be just as hard to score a perfect "10" on as any skiing or teaching task in the exam. For the rare person with that kind of superior talent, skill, or knowledge in any of these areas, the exam does provide an opportunity to demonstrate how far beyond the passing standard you are. You only need a "6" to pass!
Best of luck!