I'm aware that if I build our app on Xcode 4.5, We won't be able to submit our app to the App Store not to mention we won't even be able to run the thing on the device with iOS3.x.
Not what I want to clarify is that, if we decided to still support iOS3.x do we just use Xcode 4.3 or any Xcode lower than 4.5 and voila I can submit it to the app store and Apple won't give a care and approve the app if it supports iOS3.x or not since it was built using a lower version of Xcode?
As long as you provided an iPhone 5 screenshot, you should be able to submit app that backward supports iOS 3.x .
But why do you want to support iOS 3.x ? Not many devices are using this iOS version .