I am running version 16.04.2 version of Ubuntu and Unity Desktop and have my touch pad disabled in System Settings. My mouse has 2 buttons and a middle wheel.
When I highlight something and press the middle wheel, whatever was highlighted gets pasted as expected.
But, pasting also used to work when I press the left and right button together. This is no longer working. When I press the left and right buttons together what comes up is the popup menu which has 'Open Terminal'.
Looking at How can I paste text by pressing the left and right mouse buttons at the same time?, it seems like I need to enable middle button emulation by running the
gpointing-device-settings
command.
This command is no longer found in the latest version of Ubuntu.