Hi amitsah,
GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. Assuming a similar 'swing' in how your CATs function, your 3 CAT score results show that you essentially performed the same each time (about 600 +/- a few points). Many Test Takers who use a 'book heavy' study approach end up getting 'stuck' at a particular score level. Since your studies appear to have been book heavy, then it's possible that you've gotten stuck as well, so you'll likely end up needing to invest in some new non-book resources. You have plenty of time to study before December though, so you have some room to 'play around' a bit with your studies. Raising a 600 to a 700+ will likely take at least another 2 months of consistent, guided study - and you'll have to make significant improvements to how you handle BOTH the Quant and Verbal sections. As such, if you'd prefer to be more 'efficient' with your studies, then you would likely find it beneficial to invest in a GMAT Course of some type.
GMAT assassins aren't born, they're made,
Rich