Is there an official online Apple source for documentation for Swift 3? (Preferably that can be accessed without signing in to a developer account.) Browser searches for "Swift 3" are finding links to documentation for Swift 4 at developer.apple.com, but I am interested in the older version.
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)