Hi arnem,
GMAC has publicly stated that the Official Score that you earn on Test Day is within +/- 30 points of actual ability. With a 670, it's certainly possible that you know everything that you need to know to hit 700 (although it's also possible that you got a little 'lucky' on Test Day and that your current 'ability level' is closer to a 640). Since you have a much bigger opportunity to pick up points in the Verbal section - and with just 3 weeks of potential study time - your efforts would likely be best served by focusing on the Verbal section (while doing a bit of Quant work just to keep those skills sharp).
1) Why do you have 'only 3 weeks' to study? Are you facing any looming application deadlines (and if so, would you still apply if you did not hit 700+?)?
2) Going forward, how many hours do you think you can consistently study each week?
GMAT assassins aren't born, they're made,
Rich