APPLE INSIDER
Apple has provided developers testing its betas with the third build of macOS Big Sur 11.3 to try out.
The newest builds and configuration profiles can be acquired from the Apple Developer Center, or as an over-the-air update on hardware already running beta software. Like Apple’s other operating system betas, a public beta variant will usually arrive a short time after the developer version.Previously Apple provided the second developer beta for macOS 11.3 on February 17, with a public beta available from February 18. The first developer beta was issued on February 2.
Source: APPLE INSIDER