At the moment, 2019.3 is more of a beta test of the announced stable build of Unity 2019.4, which will be released this spring. Nevertheless, game developers can already evaluate the capabilities of the new engine version using the example of Unity 2019.3.
The main innovations in Unity 2019.3 are the readiness to work with HDRP (High Definition Render Pipeline) and the preview of ray tracing based on it in real-time.
Also, Unity 2019.3 includes improvements to Depth of Field, Screen Space Ambient Occlusion, and the shadow framework.
Source: Unity