I am in 3rd sem now so from which sem I have to take GATE coaching ?

This is very good to know that you are so serious about your Gate preparation but as per my view it is better to start your preparation or taking coaching in your 3rd year because first 2 years of B.Tech is the ideal time to make all your basics concept clear and strong. Apart from this, below I am providing Gate preparation tips which you should keep in your mind whenever you start your preparation  -

1. Know the syllabus very well.
2. Read standard books only as there are many books available in the market so it is advisable to use the books which make all your concepts clear and which are easy to understand.
3. Understand the weightage of topics properly.
4. Proper Revision plan should be followed.
5. Mock Tests and Practice Tests should be done as this will help you to learn time management skills and this will also make you familiar with the exam pattern.

Hope this helps!!!

I will suggest you to first look gate syllabus as soon as possible and then with your university exams prepare more for gate subjects by video lectures and books. Try to start learning basics. If you do these things you don't need to take coaching but if you want you can join in 5th sem