I disagree that your Solid core door needs to be "3 inches thick and completely sealed". You can get a decent STC rating (probably around 55-60) with a regular stock solid core door, and full gasketing all around. Use gasketing from the autoparts store for car doors, and you'll be fine. Trying to get an STC rating above that in a residential environment is fairly ridiculous, and would involve a triple layer, triple sealing multi-thickness monstrosity.
Be sure to gasket around the floor as well, or use a neoprene buffer.
Be sure to gasket around the floor as well, or use a neoprene buffer.