You can make your app run on iOS 8 & 9 by setting the Deployment Target to one of these versions. Swift 3.x is compatible with iOS 8 and newer (I'm not sure, but it might be also compatible with iOS 7). The only difference to Swift 2.2 (regarding the system requirements) is that you have to use Xcode 8.
When you set your Deployment Target to an earlier version than iOS 10, you should be aware that you cannot use APIs that are new in iOS 10. (except you use the #available
operator) But using Swift 3 should be no problem.
Edit: You can now upload apps written in Swift 3 using Xcode 8.0 GM
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