Chernobyl wasn't really an "accident", per se. Pretty intentional. But it is the only 1 that was officially rated as a 7 on the 7 point international nuclear and radiological event scale.
No containment vessel. Purposely started meltdown as an experiment. They wanted to "test" a new emergency cooling method. It's not like they thought it would work, they literally weren't sure. 4 previous tests didn't work, but they didn't think they had given it enough time to fully work. Giving it more time means going closer and closer to disaster. The risk to people's lives were considered worth the glory of Russia, so the decision was made to go forward, knowing the danger.
Three mile island wasn't anywhere near a major disaster. Rated a 5 on the official scale, but that has plenty of critics, as there were zero injuries or environmental effects. It was a near miss, but not a disaster.
Minor amount of low grade radiation released beyond containment, and most of that was contained in room temperature solids and liquids which were contained on facility grounds. Nobody recieved any more radiation than would occur in a common chest x-ray, and there have been no documented public health effects.
Fuikishima isn't rated yet. Likely to be a 6 or a 7, though. Aside from Chernobyl, which was just idiotic, Fuikishima is by far the worst true nuclear disaster outside of lab environments.
