Hi cemmy,
To start, a 580 is a solid initial CAT Score (the average score on the Official GMAT hovers around 540-550 most years). If this really was the first work you've done to prepare for the Official GMAT, I have a few questions about how you took this CAT:
1) Did you take the ENTIRE CAT (including the Essay and IR sections)?
2) Did you take it at home?
3) Did you take it at the same time of day as when you plan to take your Official GMAT?
4) Did you do ANYTHING during this CAT that you can't do on Test Day (pause the CAT, skip sections, take longer breaks, etc.)?
5) Did you take this CAT before? Had you seen any of the questions BEFORE (re: in an online forum or in a practice set)?
Nobody here knows you well enough to know what you are truly capable of, but the goal to raise this score 90+ points in just 3 weeks might be too difficult to be considered realistic. You would have a much better chance to hit your Score Goal if you spent another 2 months of consistent, guided study on this process.
1) Is there a reason why you're thinking about limiting your studies to just 3 week?
2) When are you planning to apply to Business School?
3) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich