Hello Prashant

You cant give gate exam during your third year of  btech.

You can write exam in final year and after.

But you can start your preparation anytime , how much earlier that much better.

You can check all details of gate using the link below

https://engineering.careers360.com/exams/gate

All the best!!

• Candidates should have completed their Bachelors degree in Engineering/Technology or Masters degree (M.Sc) in any relevant science subject or must be in the final year of the programme.