Similar to tvOS 9.1 Beta 2, the third build of tvOS 9.1 Beta is also a minor update featuring bug fixes, under-the-hood enhancements, as well as performance improvements. Although, Apple is asking devs to " test your apps with this Beta and file bug reports as needed".
tvOS 9.1 Beta 3 is now available through Software Update for registered developers. Although, to install an initial tvOS beta, it is required to have the Apple TV connected to a computer with a USB-C to USB-A cable, with the software downloaded and installed via iTunes.