What are my chances of getting into old IIMs? I have 10/10 GPA in X, 88.4% in XII, 8.6/10 GPA in B.Tech?

Forget about your GPA and CGPA first you need to focus on your CAT percentile. Ideally, you should be above 99 percentile to get into IIMs only exceptional student who have managed to get even at 87 percentile this year but they will be rarest of the rare so focus on CAT your communication, GD, PI a