A three-month period is good enough for you to get a good SAT result. A combination of the below mentioned pointers if included in your study plan can help get a great score.
• Check what is historically tested in the exam
• Practice-practice-practice and tackle each problem
• Read voraciously and build your vocabulary
• Try to make a little progress each day, make learning a practice.

Follow this regime and you will see yourself making good progress:
• Take a practice test and see where you stand
• Using your practice test as an indicator, make a personalized topic specific plan for yourself to tackle your challenges
• A little progress everyday adds up to big results, so practice every day.
• Take simulated tests to avoid any last-minute confusions. This will also give you a feel of what it is like to take the test.