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.
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Free Session: September 14: Learn how to define your GMAT strategy, create your study plan and master the core skills to excel on the GMAT. Click here to attend.