I want to extend SP2013 web application on a sharepoint server having two front end servers and one back end server. How to do that? I have extended one web application but it was on sharepoint server having one front end server. Is it the same way we have to do? or we have to also extend the web application on another front end server.
Extend web application as you did in 1WFE farm. You don't need to do anything on each WFE. But if you've got web applications provisioned as HTTPS, you'll need to take the appropriate measures to get the SSL cert configured on the servers, that is something SharePoint won't do for you.
For more info look at this guide: Extend web applications in SharePoint 2013
Think about host names, sometimes it is better to have.
And also take a look at this answer: SharePoint WebApplication publishing