3 weeks is a very short period of time. Still, you're not looking for a crazy score improvement in that time, so you might make it. Just know that you're going to have to work hard!
The first task is to figure out your starting strengths and weaknesses, so that you can pick out the "low hanging fruit" (the easiest things to improve).
First, read this:
http://tinyurl.com/executivereasoningThink about how what you've been doing does and doesn't match up with that and how you may need to change your general test-taking approach accordingly.
Next, use the below to analyze your most recent MPrep CATs (this should take you a minimum of 1 hour):
http://tinyurl.com/analyzeyourcatsBased on all of that, figure out your strengths and weaknesses as well as any ideas you have for what you think you should do. Concentrate on the easiest things for you to improve: careless mistakes, things that you were good at in high school but have just forgotten, etc.
Then come back here and tell us what you think; we'll tell you whether we agree and advise you further. (Note: do share an analysis with us, not just the raw data. Your analysis should include a discussion of your buckets - you'll understand what that means when you read the last article. Part of getting better is developing your ability to analyze your results - figure out what they mean and what you think you should do about them!)
Also, what study resources do you have available already? You may need to buy some books / other study materials that target the areas that we decide are the best ones to improve.