What is the offical name of the third on-screen button? I am searching for the official name of the third (right) android on-screen button.

The left and middle button are clear:


Left: Back button
Middle: Home button
Right: ???


Android 1 - 4.4

Android 5.0 - ?


Please include a reference where the name of the third/right button is stated in your answer.
It's called Overview button. I originally came to know about this  from the Screen Pinning settings' description.
Google on Get around on your device (see Get around with navigation buttons) has noted:
Overview: Opens a list of thumbnail images of apps and Chrome tabs you’ve worked with recently. To open an app, touch it. To remove a thumbnail from the list, swipe it left or right.

It looks like this


If your device is running Android 4.4 or lower, your Overview button looks like this