Developers taking part in the program can download the latest test build via the Apple Developer Center, or receive it as an over-the-air update on enrolled devices. Participants in the public Apple Beta Software Program can usually expect a similar beta update to be issued shortly after the developer version.
The third iOS 12.3 beta is build number 16F5139e, replacing the second build, 16F129d. Apple’s third build of watchOS 5.2.1 is build number 16U5101c, up from the last version, 16U5091b. The new third build of tvOS 12.3 is build 16M5139a, taking over from the second build 16M5129d.