Hi manojee21,
A couple of comments.
First, you should only trust your first two GMATPrep scores. I know this might be discouraging because you scored above your goal on the third exam, but the problem is that GMATPrep is only meant to be taken twice and you could have seen questions on the third test that came up the first two times around due to the limited question pool. This would definitively skew your score UP.
Hence, you should only be looking at the following scores for comparison:
1 Gmat prep test - 600 (Q-42,V-32)
2 Gmat Prep test - 690(Q-49, V-35)
Manhatton[sic - we're spelled "Manhattan"] - 640 (Q-42,v-34)
Hence, your score range is between 600 and 690, making the 640 you received on our test about on par with your performance. What's good about your scores is that your verbal performance is fairly consistent - between 32 and 35. The wild card in your score, however, is your quant score. I'd suggest focusing on making fewer careless errors in this section (you are clearly capable of scoring high on the test based on your 2nd GMATPrep test) and then further solidify your verbal skills before test day.
As for extremely challenging math questions, you can try our Challenge Problem Archive. Please note, however, that some of these questions are SUPER difficult and may even be outside the scope of the GMAT. However, they could help you critically think about the math section in a different manner. I haven't been a GMAT Club member long enough to post URLs, but you can visit our website and buy the Challenge Problem Archive in the store.
That's about it - let me know if you have any other questions.
Best,
Michael Dinerstein
Manager of Online Marketing
Manhattan GMAT