This third needle is known as the "mid screw". I would recomend finding out what the factory setting is for the low and mid screws and re-tuning from there. The reason for the mid screw is to allow the racer to change the poing where the low needle "turns off" allowing full flow from the high speed needle. If you were to want to adjust the mid screw, you would want to turn the low needle the same amount in the oposite direction of the mid screw. i.e., if you want the move the mid 1/2 turn out you must also turn the low 1/2 turn in to keep the amount of fuel the low needle allows through.
Hope that helps.
"I have no idea what you're talking about, so here's a bunny with a pancake on it's head."
In mind and body, we're all heading towards old age. It is slower to go by bike.