Where to find online documentation for older versions of Swift?
If you're using a modern Apple Device, which you must be in order to develop with Swift 3, you can use iBooks' Swift 3.1 Guide. Once Xcode 9 and Swift 4 are finalized, this book may receive an update to Swift 4, but you should be able to decline the update.
Check out the book here: The Swift Programming Language (Swift 3.1)