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.
Do you want to learn how to pre-think assumptions in GMAT CR Arguments? We have an upcoming session on the 31st of May 2015 at 7 AM PST on How to Pre-Think Assumptions. Click here to register for this Free Session.