You can get the three-button navigation bar back on Android by opening Settings and selecting 3-button Navigation under ...

Google may be testing the option to customize Android's navigation bar to add a little more than what's currently available.

Google is preparing some new features for Android’s desktop windowing mode, including a button to minimize apps.

That is the 1Tap Quick Bar app displaying no icon, and allowing the app to display information in the shade itself. One of the most useful shortcuts this app lets you to add is the redial button.