hello,
first i advice you to take the gmat official practice exams(they give you 2 exams for free upon registration on mba.com). gmat official practice exams are the closest simulation to the real exam.take one of them to see an accurate simulation of your real score and keep 1 of them for later after you finish your prep.
second step : a lot of practice.
e-gmat courses are good but i strongly advice you to buy the official gmat books too because as i said no one knows gmat better than gmat official makers

.
3 month is enough for preparation but you have to study 4 hours/day.
final hint: check gmat club mock tests, they are harder than the average gmat question. believe me you will find the gmat questions easy after you practice the hard stuff
gmat club tests provide.