Chernobyl was a major disaster, no question, and it was pretty close to as bad as it can get. Not really surprising when you consider that it was engineered by the Soviet Union, a system not noted for its concern for the people.
But 3 Mile Island, while scary, was NOT a disaster. The system worked there, the leakage was absolutely minimal and the safeties stopped any real problems. I read somewhere once that the exposure received by those who worked at the facility and those who lived nearby, was actually less than the average normal exposure that citizens of Denver receive from solar radiation. Yes, it could have been worse, and mistakes were made. But it was far from cataclysmic.