Can an average non IITian make it into IIM Ahmedabad in the first attempt with three months preparation?

Yes, it may happen by chance. Miracles do happen but it’s not likely.

Average means different to different people. But what I feel is that an average student means a student who lacks spirit, confidence and does not have the knack to crack an aptitude test. So it is going to be difficult for an average guy to crack IIM Ahmedabad.

For converting IIM Ahmedabad you need to master the CAT test-taking method which takes time. It is beyond just simply covering the syllabus and this will take a minimum of around 6 months for a normal student. Of course, there are always extraordinary talents who have such a strong grasp on Mathematics and English subjects that they need only 3 months. But since you are an average guy, you will need more than just three months of preparation.