If you are an Apple registered developer, then you would be interested to know that Apple has released first developer betas of all its operating systems and they are now available for download through the dev center. Given that Apple has just released the major software updates for all these operating systems it is very unlikely that these betas will bring any major changes or introduce any new features to iOS, macOS or tvOS. They will mostly focus on bug fixes and performance improvements.
If you want to download these betas on your devices, then simply log into your developer account and get the beta files. macOS Sierra 10.12.3 beta 1 is available as both standalone file and through software update on registered devices. iOS 10.2.1 on the other hand is only available as an over-the-air update. The IPSW file is not available for download yet. Users can download tvOS 10.1.1 beta over-the-air as well as through iTunes.
If you are a developer testing these betas, don’t forget to mention if you find any noticeable change in this release.