If it's a constant rotation pot then it'll be giving analog values.
Measure the resistance between the green and black. That should be constant, no matter the position of the cog.
Then measure the resistance between the white and green, and turn the wheel. The resistance should change.
Do the same between white and black - the resistance should change - in the opposite way.
With the cog static, the resistance between white & green, and between white & black, should add up to the same as the resistance between green & black.
If that is so, then connect the green to +5V, the black to GND, and the white to A0. Read from A0 to get the position of the cog.