The three legged LED is the equivalent of two LEDs connected back to back.
Kind of like this, but in one LED. (Someone correct me if I'm wrong.)
(x and y voltages are the same, but seperate circuits. So the LED can show two/three colours depending on which side(s) is/are lit.)
Code:
GND -----|<|---+---|>|----- GND
|
|
+VCC
(sorry about the craptacular ASCII art, but that's all I can come up with on short notice.)
So to replace it with one LED, you would have to figure out which side of the LED is powered for 'Power On' and which side is powered while the unit is in standby mode.
Then you would connect the 2 legged LED in the holes where the pins for the 'power on' side of the LED were.
(Sorry if I explained this badly, I'm tired. (Changing phase the hard way.))
EDIT: Took me that long to bang this out that scotty got there first. Ah well.