AFAIK, in Xcode 4.5, armv6 support has been deprecated and the deployment target should be minimum iOS 4.3.
Therefore, it is not possible to develop an application which works on iPhone 3G and iPhone 5. Is that correct, or am I missing something?
That’s correct. Well, you could build your app with older Xcode so it will work on both iPhone 3G and iPhone 5, but not use up entire screen on the iPhone 5. This was intended, iPhone 3G support is long gone.
Edit: there is an unofficial solution provided as an answer here: How to support both armv6 and armv7s for release build in xcode 4.5