Where is the Shift Solenoid D and how do I change it? What is the Hyundai part#?

I would recommend a qualified repair shop perform a diagnostic on your transmission. Stuck in 3rd gear is usually a default mode for any transmission electrical problem such as a solenoid, actuator or controller ( Computer). Could be a solenoid but why guess and waste money. I feel it is always best to know what the problem is before spending money on it.