My website is really very simple HTML based site. My site is located here: http://www.threesite.com
How can I convert this simple HTML site compatible with iPhone or Droid? Do I need to convert the same into PHP based site and then convert? Your help will be much appreciated.
5 Ways to Create or Convert into Mobile Phone Website.
The easiest way is to use Google's tool, because it converts your existing site automatically. I've tried it on your site and it looks nice.
If you want to tailor the site yourself, you have two options: Building an app or rewriting HTML. A site app makes your site highly customizable and professionally-looking, but it requires a lot of effort and separate development for each platform (iPhone, Android, Blackberry, Symbian, ...), and. it requires your users to specifically download your app.
If you choose to convert the existing HTML to a mobile-friendly site, it will be (mostly) cross-platform, and you can follow the guidelines here.