iMessages on iPhone 3G?
4.2.1 is officially the latest supported version for the iPhone 3G, but you can supposedly install iOS 5 on it with this method: http://osxdaily.com/2012/01/19/install-ios-5-on-iphone-3g-with-whited00r-5/

In the video, it shows the Messages app on the homescreen, so that ought to work for you.