What Swift Version Should I be using?

I have a few questions related to Swift Versions.

I am currently writing an app in Swift 2.2 I am planning on releasing the app to the App Store before iOS 10 is released. Does that mean I need to stay using Swift 2.2 until iOS 10 is released?

Once iOS 10 is released, if I upgrade to Swift 3.0, will I still be able to build for iOS 8 and 9?

What is the purpose of Swift 2.3? Overall, I'm trying to understand what the optimal time to be upgrading versions of Swift is while maintaining compatibility with older iOS versions.