The third screw with a nut on it is a idle stop or idle speed and throttle barrel retainer screw. The tip of the screw fits into a spiral slot in the throttle barrel so it rotates in and out as it is turned. Most people use the TX trims and idle stop settings for controlling the engine idle. So using it as a idle stop isn't used much anymore. Since it isn't mentioned in the instructions it is really just a barrel retaining screw then. Nothing to tinker with unless you were disassembling the carburetor for some reason.