The traditional "show desktop" button in the task bar of GNOME is gone since 3.x version. This is extremely inconvenient when we want to see the files in our desktop with multiple windows opening. However, a gnome-shell-extension, known as"minimize all",  can minimize all open windows and show us the desktop. Our conveniences come back again with this useful tool.

You can obtain this useful tool from here:

https://extensions.gnome.org/extension/760/minimize-all/

The version 3.20 works fine under SUSE Linux Enterprise 12 SP3 OS.