Personally, I think the most selective schools (10% acceptance rate and below) are looking for 750+ on each section for their unhooked candidates. That is what I have observed at my son’s school. The biggest cluster of naviance acceptances seems to come at 2300+ and even better 2350+.
However, if you have already taken the SAT three times with similar results on CR, not sure you can move that much higher unless your weakness is vocabulary based, which can most easily be rectified in a short amount of time.
Edited to note that this mainly applies to HYPSM. I think your scores are in good shape for Northwestern, but check to see where 700 CR falls in their specific range.