I have created a project with the main page (index) and two duplicates which are "about us" and "contact us".
How can I save all these three pages in one single folder as the website should be ?
The pages don't save to a folder they save to the root directory of the website "as a website should be" lol. You "can" create folders for your pages same as you can for images, css, etc., but I don't know that if the Linked Components will respond correctly. One thing I noticed is that no matter where I put a page, if I update a linked component it will update it in that page as well and I haven't delved far enough to see if it is adjusting the links to accommodate the folder the page is in or not, you'll have to try it. I have only ever added folders as places to put pages that I wasn't working on right now so I didn't have to deal with them in the list of pages so I've never cared if they updated the links, hence why I don't know :)
Other than that, create folders by right clicking over the Pages section title and choose folder, then move your pages there and you're all set. If you use linked components and check this out further please post your findings if you would so others can know including myself.
Hope that helps!