Creating all three platforms is then as simple as copying the HTML from OS X to Windows, or vice versa. Most developers I know either have a Mac with Windows installed as a Bootcamp or a Mac running Windows in a virtual machine like VMware or VirtualBox.
Android (built in Java, essentially) can be built anywhere.
For Windows Mobile, you must build your PhoneGap application in a Windows environment. One caveat here is if you want to build IOS applications, you have to have your PhoneGap installation on a Mac running OS X and Xcode. In this article we are going to look at how to install PhoneGap, using the command-line interface, on Windows and MAC OS X. This means that you can access functionality formerly only available to the native device language using JavaScript.
PhoneGap even gives you plugins that allow you to access your device Camera, Compass, Contacts, GPS and much more.
The idea is simple - you build your application in HTML5, and then add that code to your PhoneGap application, and it compiles to Android, IOS and Windows for you. PhoneGap has quickly become a popular platform for building mobile applications, mainly because of the simplicity involved in creating cross-platform applications using only HTML, CSS and JavaScript.