How should GATE be cracked within 3 months ?

The answer for that question solely depends on You.

Here are some points that would help you :

1. Analyze Yourself

I am just a random guy on the internet,you can't. solely depend on my answer. First try to analyze yourself. Go through the syllabus and make a note that how much good you're with your syllabus. You can find the amount of syllabus you've covered by this prepmeter.
Analyze your preparation level

2. How Good are you?

Now you have gone through your syllabus and can know your weak topics.Try to focus more on them.Try to allocate as much as time as possible for those topics.And be intelligent enough to leave the topics that aren't necessary.

3.Follow Standard Textbooks

I can't stress that enough.Majority of my friends who are preparing for GATE exams bought some Mock test papers and were trying to solve them.They are more interested in knowing the technique than learning the topic.They all stuck with all the topics and were solely relying on the
"key".Don't be such kind of a person.Learn from standard textbooks they help you in clearing your basics.

Education is like a house,the basement has to be strong for the house to cherish.

4.Connect Everything

I once read a quote,it says like

" Education is learning random things.Knowledge is about
connecting all those things".

All your subjects are related to one another. So if you read a subject well then it will help you in understanding the other subject. Study SOM before MD. (Sorry! I'm a ME guy).

5.Practice.

Practice and Practice even more Lock up yourself in a room.Study a subject and solve all the questions regarding that subject in previous papers.There will be some topics in which the IIT's concentrate more.Find them and practice them even harder.If you're in college,Read the topics at home and go sit in the last bench of your class and try solving those questions. Never stop practicing.

6.Success comes at a pay

As the GATE exam is about to approach just try to give more time for your preparation.There will be lot
of time you would have been wasting in your day. Just try to minimize that.
DON'T USE INTERNET UNLESS IT IS ACADEMIC ORIENTED.(You can have
a little relaxing time on it but make sure it doesn't count as a distraction).

Give all of your effort.