There are 4 temperature sensors in GPU core and temp #3 is one of them. Remove the heatsink, clean up the old paste with a Q-tip, apply TIM (MX-3/MX-4) then check for contact surface. If all good, clean it and apply TIM, put everything back on and it will show 4 even full load temps.
That's highly misleading. Even the creator of GPU-Z said that he doesn't know what each of the temp #'s refer to as it varies from card to card and some times a temperature will be reported for a sensor that doesn't even exist on that card.