I am sorry to know that you could not achieve you target score even after 3 attempts. How did you prepare for the exam. Regarding strategy for 680, I recommend that you view the recording of my strategy session below.
Recording link: Click here
You will get to the recording as soon as you register.
Learn how eGMATers improved their GMAT scores by 100+ points - Click here
Learn about our newly released Quant Online course here.