How can I show my desktop in Gnome 3?

You can create a keyboard shortcut to show your desktop. In the upper right-hand corner, click on your username, then select the System Settings icon (looks like tools). Then Click on Keyboard. On the left-hand side, select Navigation, then on the right-hand pane, look for "Hide all normal windows". This should say Disabled. Click on it, then it will show "New accelerator". Enter the keyboard combination you would like to use to show your desktop. The most common shortcut for this is Control + Super + D. Super is usually the Windows key. Now, when you type Control + Super + D, it should toggle between showing your desktop and showing the windows.