Hey, sorry it took a while to get to your message....
I'd suggest your case is a pretty good one.
Improving math takes less time than verbal. I assume you have done some math preparation, so you pretty much know the basics and I will omit those.
Since you have 3 weeks, I would suggest concentrating on Arithmetic and Word Problems - these account for about 70% of questions on the GMAT. (my guess). A lot of data sufficience is Arithmetic. Word problems dominate the PS section. Arithemtic, actually, is very often underestimated - one needs to brush up and learn all those little properties of numbers, and zeros, and negatives, etc. THey are the basics. A good source would be Kaplan
's Math Workbook. The review sections are very short and are follwed by a large number of exercises. If you don't have time to spend solviing all of them, here are the numbers of the questions you should do for sure. (the intense course)
's Math Book 2nd Edition.
Number properties test: 8, 13, 14, 17, 19
Averages test: 4, 6
Percents: 4, 7, 8, 22
Powers and Roots: 5, 12
Algebra test 2: 2, 6
Basic Word Problem Exercise (p. 131): 6, 8, 9
Word Problems Test 1: 4, 19, 24
Percents, Ratios, and Rates: 34
General WP Test: 31, 35, 40
Triangles Exercise: 7, 11
Triangles Test: 2, 15, 18
Quadraliterals and other polygons: 14, 16, 17
Circles: 2, 5, 10
Multiple Figures: 2, 6, 9, 14
DS1: 12, 14, 16, 17, 18, 19, 20, 22, 24
DS2: 3, 7, 9, 10, 15, 21, 23, 24
This is for the second edition. I think the third is the second with a few errors corrected, yet I don't guarantee it will be exactly the same. In any case, I do recommend this book a lot. I have used it for my prep.
ONE THING THAT YOU SHOULD DO FOR SURE: Review the rules and properties. Make sure you know the theory before you jump to solve the problems. Don't try to take GMAT sample tests now, concentrate on specific topics.
On this site I have Arithmetic 1 Posted, you can check it out if you have time. Here is the link: http://www.gmatclub.com/members/courses/quant/arithmetic1.shtml
Also, time management on GMAT: http://www.gmatclub.com/members/courses/quant/timing.shtml
I will also email you a few selections I have made while tuturing.
It will be great if you can get in.
Does anybody else have any suggestions for math?