Beta testers who have signed up for Apple's beta testing program will receive the 10.12.3 macOS Sierra beta through the Software Update mechanism in the Mac App Store.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to both iOS and macOS Sierra betas. Betas should not be installed on a primary machine due to the potential for instability.
According to Apple's release notes, the 10.12.3 update "improves the stability, compatibility, and security of your Mac." No specific changes, bug fixes, or feature additions were discovered in the first two developer betas.