Usually, I would use expo on the throttle to make the engine response linear. If the linkage and the EPA (ATV) is adjusted properly, the response should already be linear. If you want the throttle response to slow down, there is a mix in some radios called throttle delay, and that will slow down the servo controlling the throttle, but I highly doubt that you need it. With 3D, you want the fastest engine response possible to get you out of trouble.