is it possible to use a USB hard drive with the iPad 3? Do I need to download an app to use it (assuming I have the camera connection kit)?
You can with the iPad Camera Connection Kit, but in a very limited manner. You can import photos or videos on an external drive to the Photos app, if you set up the file structure to emulate a USB-connected camera - I believe you need a folder in the root directory named
You may find this article useful: CNet: Secrets of the iPad Camera Connection Kit