Three weeks is definitely not much time to prep yourself for a 100 point improvement. If you're really in a crunch, I might consider hiring a private tutor to specifically address your needs and improve upon your weaknesses.

It's not about how much time you study or how many questions you can cover in your prep, but rather how well you structure a smart study strategy with your remaining time. Have you performed deep analysis of your errors in the past? I recommend taking the time to look at the resources available here: https://www.beatthegmat.com/resources.html. There are a lot of links and resources there to help you figure out an efficient prep time.

Otherwise, understand that it's going to be really difficult challenge to get your target score given the time you have.

Good luck!

As Eric said, it's highly unusual to improve 100 points, particularly from a 600 to a 700, in only a few weeks. Even hiring a private tutor may not be enough - it's just not enough time.

You haven't mentioned what kind of deadlines you may have with respect to applications - but, if you can take the time, you should.

Most people wanting to go from a 600 to a 700 study for about 2.5 to 3 months. (This assumes that you haven't already been studying for months to get yourself up to a 600 - it's easier for someone naturally starting at a 600 to get to a 700 than for someone who started at, say, a 500 and has now gotten themselves to a 600.)

Also, as Eric said, the quality of your study is much more important than the quantity. You have to make sure that you are learning all you can learn from any problem you do before you move on to a new problem. You also have to understand where you are falling short, in terms of both knowledge of content and test-taking methodology, so that you can combat those weaknesses. Extensive analysis is key - look at Eric's resources for help.