It's going to be very difficult to make that improvement in 3 weeks, especially if you are busy with other things. If you had been studying systematically and just needed some practice to put it all together, that would be one thing. (For instance, some people make great strides with the material, but get into big trouble with timing on the practice tests.) But if you still need to get stronger at the material and learn some new approaches, that takes time. Getting to another level on the GMAT is about training yourself to respond differently to the material. For instance, to improve on rate/work problems you might need to improve your ability to recognize what kind of scenario the problem is testing, set up a chart or system of equations, and then solve efficiently. To improve in RC, you might have to change your reading style to focus more on the author's opinion and passage structure. You can certainly cram in a lot of ideas over 3 weeks, but it's hard to say that those will lead to a consistent change in approach. Many people revert to their old methods under test pressure.
So if you want to shoot for your top-choice school, that December date may be too soon. Then again, schools will generally only look at your top score, so if you want to give it a shot in December, it's not going to hurt your future prospects. In the end, use your practice CATs as a guide. Don't go in expecting anything higher than what you're getting at home, and that's assuming your tests are complete and realistic, with essay, IR, and official timing.
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