help! gmat in 3 weeks, varying practice scores

Congrats on the good score! The practice tests you get from signing up the exam are indeed the very best, most accurate exams out there. I bet you'd be surprised at how many people do well with little or no preparation. They go to Wharton and Stanford and work for hedge funds .

For someone coming in cold, the "low hanging" fruit of improvements are usually the benefits that come from being more familiar with DS and some number properties. I'd continue to practice rather than read a book and you'll do great.