I'm not a math person either, and I also only had 3 weeks to study for the GRE -- but ended up making over 700 in both sections! I liked the Barron's book for vocab, though the math was much more difficult than what you'll encounter on the GRE. (At the same time, though, mastering Barron's math made GRE math feel like cake.)
DON'T cram. Don't study for hours and hours every day, because you'll burn yourself out. I studied 3 weeks for 2-4 hours a day, and not at all for the two days prior to the test. There's a point after which you'll stop retaining new information, and I think that point is after about 4 hours of studying. And don't make that 4 straight hours, either. Study for half an hour, break for an hour. Repeat. Do lots of practice problems, and don't do Powerprep until a week before your test.
You'll be fine! Mastering GRE quant isn't about being good at math, it's about not freaking out, being patient, and trying to work through logic puzzles. You can do it!