Hi bratbg,
Many Test Takers spend 3 months (or more) of consistent study time before they hit their 'peak' scores, so since you've been studying for just 2 months, then that might help to explain why you have not scored higher yet.
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 (600 +/- a few Scaled Score points). You handle certain aspects of the GMAT consistently well, but you also make certain consistent mistakes. Raising a 600 to the point that you can consistently score 700+ will likely require 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. Thankfully, the GMAT is a consistent, predictable Exam, so you CAN train to score at a higher level. By extension, you should consider paying the re-scheduling fee and pushing back your Exam. There's no harm in taking the GMAT as scheduled, but you can save some money and aggravation by rescheduling.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
1) What type of study routine have you been following? How many hours do you typically study each week?
2) When are you planning to apply to Business School?
3) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich