Hi,
Joining a good coaching is definitely necessary for a sequential and planned study/preparation for GMAT within a short time span like 3 months, specially if you are newbie in GMAT field.
But before taking a decision you take a Mock test. This will let you know your strength areas in GMAT, along with a good exposure to GMAT question pattern. Based on your analysis after taking Mock test, you can choose for the area (Verbal or Quant or Both), for which you need a coaching. Before giving the Mock don`t forget to brush up your basics from any good resource (books, URL etc) for GMAT.
Best of luck!!!