Hop over to swift.org to download the first Swift 3.0 PR 1 installer. Double-click to install, authenticate as admin. Once you do, the toolchain will appear in the Xcode > Toolchains submenu.
When you select a new toolchain, Xcode needs to quit and relaunch. It will ask you to confirm that’s what you want to do. Give it a few seconds to catch up with itself.
3.0 PR 1 is not compatible with playgrounds. The easiest way to play around with code is to create a Cocoa command-line app (or, if you really need to, use a single view UIKit app, but it’s a lot more complicated) to try out the new syntax, test libraries, and get a feel for what’s new and changed.
Now that the toolchain is readily available, I’ll be posting highlights of new features when I get some time.