Unacceptable CAT scores with 3 weeks left

Well, your question is sort of like asking: right now I can run a mile. How many miles will I be able to run in three weeks?

The answer, of course, depends on many factors, including:

1. How often you practice
2. How effective your practice is
3. Your abilities and potential
4. Whether someone is helping you (e.g., a coach)

The GMAT is no different. You might improve 100 points, or you might not improve at all. It depends on how much and how effectively you study, your general aptitude for the GMAT, and whether you take a prep course or hire a tutor. As a baseline comparison, the average score improvement for a Veritas student is slightly above 100 points. This is the average improvement over a two-month course, including all lectures, homework, and practice tests. So, you can probably expect something less than 100 points in three weeks of prep, especially if it's self study.

You've probably read this already, but GMAT Prep tests tend to be the most accurate, so you're probably somewhere in the high-500s right now. I'd think an optimistic but likely attainable goal would be mid-600s.

Good luck!