A few days after the latest iOS, iPadOS, tvOS and watchOS betas were released, Apple has made the macOS 10.15.3 beta 3 available to developers for testing.
The newest macOS Catalina beta can be downloaded via the Apple Developer Center for those enrolled into the test program, or via an over-the-air update on devices running the beta software. Public betas typically arrive within a few days of the developer versions, via the Apple Beta Software Program website.
It is unknown what Apple is changing or updating in any of the betas, as the support documents for each version state there are "no release notes" for the software update. The incremental nature of each strongly suggests the betas are for performance improvements and minor changes, rather than relating to the addition or alteration of features.
Both AppleInsider and Apple itself strongly recommend users don't install the betas on to "mission-critical" or primary devices, as there is the remote possibility of data loss or other issues. Instead, testers should install betas onto secondary or non-essential devices, and to make sure there are sufficient backups of important data before updating.