You already wrote the answer on the title. You can use ajax. I suggest you to go to an ajax tutorial page and learn how it works. Because what you want to do will be an example on that page. Anyway let me explain what you will do. when your links clicked instead of linking your page you will call a javascript function. function will goto server and bring content of the page in string format. and you will put your content in your page like following
document.getElementById('mainDivElement').innerHTML = responseText;
if it seems comlicated. make 3 ajax example. read this again. you will understand everything perfectly.