You haven't given me a lot of details, but 3 months for a 400-point score improvement - most people would need more time. With that kind of a score improvement goal, it's not even really possible to say right now when you'll be ready to take it. You may be ready in December or you may not be ready until March - it's too far in advance / too far from the goal to be able to predict with any real certainty.
Start here:
http://www.manhattangmat.com/blog/index ... an-part-1/Make sure to read both parts of the article. This will tell you both what kinds of materials you need to get and HOW to study.
I'd also like to get some more detailed data on your strengths and weaknesses in order to advise you. Use the below to analyze your most recent MGMAT CAT(s):
http://www.manhattangmat.com/blog/index ... ice-tests/Then come back here and tell us the results of your analysis and what you think you should do based on that analysis. (Note: do share an analysis with us, not just the raw data. 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!)