Hi senthil681972,
First off, you've been studying for less than 2 weeks, so you shouldn't expect to be an expert (or consistent) just yet. Many Test Takers spend 3 months (or more) on their studies before they hit their 'peak scores', so it's highly likely that you just have not put in enough time and effort yet. Based on the data that you've provided, I have to assume that your current skills would earn you a score in the low 500s on the Official GMAT.
Taking 3 CATs in the span of 1 week was NOT a good idea. As a general rule, you should take one FULL-LENGTH CAT (including the Essay and IR sections) every 1-2 weeks. The bulk of your time should be spent on studying, learning/practicing Tactics, review, etc.
With a tentative Test Date in early December, you have about 5-6 weeks of study time remaining, which might not be enough time for you to hit your score goals.
1) When are you planning to apply to Business School?
2) Do you know which Schools you plan to apply to?
GMAT assassins aren't born, they're made,
Rich