If you're signed on the Apple Beta Software Program, you can now install the latest beta of macOS Sierra on your Mac even if you're not a developer as macOS Sierra Public Beta 3 just went live minutes after the Cupertino firm posted the third public beta of iOS 10.
macOS Sierra Public Beta 3 contains the same features as the developer-only macOS Sierra beta 4. Developer-only fourth betas of iOS 10, macOS Sierra, watchOS 3 and tvOS 10 were pushed yesterday.