With 84% in 10th, 79% in 12th, a 7.89 CGPA and 3 yrs of work experience, am I eligible for the IIM Shillong and IIM Udaipur? I am a GEM.

You are eligible for both IIMs. Your academics, on the other hand, are mediocre, which can work against you.

IIM Udaipur will contact you solely on the basis of your CAT score. IIM Shillong, on the other hand, may or may not. IIM-S has stringent admission requirements.

My only piece of advice is to ignore everything else and concentrate on getting a high CAT percentile, preferably above 99.5, and you'll almost certainly get into one of the IIMs.

With a balanced sectional and good percentile in CAT, you should get calls from both these institutes. Also, prepare well for the interview. The interview part of admission is the most difficult, so try to be confident as much as you can.

If you are confident that you will get a great percentile in CAT, you can also apply for FMS.