On a 3-terminal bench power supply (+, -, GND), are there positive and negative drivers behind each of the terminals?
This type of power supply only has one regulator placing a settable voltage between the + and - (red and black) terminals. 

The green terminal ties into the safety GND of the power cord and hopefully to the earth grounded network in your building. 

In operation of the power supply you have the choice of a fully isolated output or you can tie one or the other of the + or - terminals to the green terminal if you want to have the supply output referenced to the safety ground.