I think the only way to get 100 point improvement or close to that (all in verbal) is to spend 6 hours a day studying for GMAT. So, will be pretty tough. If you can - do reschedule. If not, here is what I can think may work:
1. Improve math a bit to get 48 - this will give you about
10 points and may be fairly easy since your math is pretty good already. May have the best ROI of your time.
2. The other 80 will need to come from verbal - focus on what you can accomplish the most as 3 weeks is not enough to cover everything In detail:
A. Pick SC, RC, or CR as your best area and improve the one that has the best ROI. Hopefully this will give you
30 points.
B. When your brain is dead after a day of studying - READ fiction books such as
GMAT Fiction - this will take a while to kick in and notice the effect, but it will be powerful enough if you read 5-10 books on that list. I would estimate
20 points here across the SC and RC
C. Make sure you work on the other two areas that you did not cover in A. You cannot neglect them since GMAT is adaptive and you never know which questions will be in the first 10-15 batch. Let's say
20 points here.
D. Start taking full length tests the week before every day - this will build up your stamina and test taking abilities -
20 points.
Total: 100 points (20 math and 80 verbal)