This isn't Pixhawk related, but ArduPilot related as the logging is done by ArduPilot. Unfortunately the dataflash logging regularly changes as we add new messages or change existing ones to add or change fields, so the documentation gets outdated fast.
Regarding the asked parameters, they are, respectively, throttle input, throttle output and throttle hover. Input and output are the throttle requested to the attitude control and output produced by the motors. The hover is the throttle needed for the vehicle to be hovering.