I want to insert page numbers in my document. The problem is that I want the page numbers to start from 3rd page but its not working whenver I apply command of "insert page numbers" it always start from the beginning.

Are you doing a project? What is there on the first 2 pages that don't need page number?
A cover? Table of contents?

What you need to do is insert a section break after the 2nd page. You can then set your page numbers to start from the second section.