Yellow is tach (RPM feedback), black is ground and the 3rd one is power supply. I say 'the 3rd one' because in most cases is red, but you're mentioning green wire. Supply wire is the middle one on the fan jack, so make sure that's green indeed in your case.

If that's the case, to make your fan run @5V, connect the green wire from your fan to the red wire from PSU and black for fan w/ black from PSU.