How can I prepare for Gate 2019 while studying in B.tech 3rd year and what are the books to be referred?

Here are some preparation tips for GATE preparation you can prefer.

Understand syllabus properly: You must be aware about the GATE syllabus, released by the organizing IIT. You must go through the section-wise and topic-wise syllabus of your chosen branch in order to know what all you have to study and what to avoid.

Solve previous year papers: Solving previous years papers is of utmost importance if you want to excel in the exam.

Weightage of Topics: it is advisable and important to study the complete GATE Syllabus, however, you can do a small exercise to gauge which sections, topics are much more important in terms of weightage given in GATE.

Resources other than books: It is good to look up topper and expert tips in this regard as they do mention the books or material they have referred to. Many GATE toppers run their own Facebook pages or blogs where they dispense advice and tips to wannabe GATE aspirants.

Remember the following points:

Divide the Topics broadly into Strong, Weak and to be read properly.

Next schedule your preparation such that you revise the easy ones along with the tough ones and the “never read before” ones. This will ensure you have a clear view of what is to be studied and how you are going to do it.

While many advise to check the weightage and study accordingly, it is really not the correct way to go.
Check the weightage by all means and understand the important topics but don’t neglect to study.
Make Short Notes while you study as this is will be helpful during revision.
Write down Formulae that you find difficult to remember so that they are handy for revision.
Keep the solved problems separately indexed so that you can refresh and revise later when required.
Analyse your preparation at every step.

You can refer the below given link for GATE preparation books as per the branch.

https://engineering.careers360.com/articles/what-are-best-books-for-gate-preparation

Good Luck!