As Jollyrog says, DO NOT take the third exam until you are SURE you will pass!!!!!
I'm using Airquiz and the PPL Perfector to help. These are practice papers. Airquiz is fantastic. Once you have paid your £20 you can practice all subjects ad infinitum, but DON'T jsut learn these questions, take a paper, see which areas you failed on, and study those in greater depth, just build on it and you'll get there. Do not go for your exam until you are getting at least 80% in your practice papers.
The PPL perfector has actual exam paper questions to practice on, and so that you know the type of thing to expect, but again DON'T just learn these. You still need to read Pooleys or Jeremy Pratt, whichever you have. Make notes as you would a school subject, then filter these down to the barest of key facts on keynote cards. This is how I study, and have passed three of my exams so far very well (including air Law).
Also, ask your instructor, thats what they are there for! Discuss your weak points and he/she will help you. Most of Air law is common sense. As I read on here once, break it down into chunks....
Vital for when airborne
Handy for when one is airborne but not crucial
Not necessary for actual flying but nice to know, and actually, I could look that up in a book if I were to need to know
Hope this helps and good luck
GQ