There were barrels with three.. eg lenovo, two charger generations ago: https://i.imgur.com/Wn4EM33.png https://i.imgur.com/Wn4EM33.png But usually the inner

There were barrels with three.. eg lenovo, two charger generations ago: https://i.imgur.com/Wn4EM33.png https://i.imgur.com/Wn4EM33.png But usually the inner one was only connected via a resistor, to tell the device the wattage of the charger (but could be used by a bunch of 1 wire protocols instead.