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Some schools may also require you to take one or more GRE Subject Tests. The purpose of each GRE examination, of course, is to help graduate schools decide if you've got the right stuff for their program. If you're looking for help with the exam, check out Peterson's full-length GRE practice test.
Yes, 3 months are sufficient to prepare for the GRE, provided you have a basic proficiency in English language and Quantitative ability sections, respectively. For the verbal ability you must adopt a two-pronged strategy. Primarily, you should try and bolster your vocabulary.