I know next to nothing about nuclear power, and as such, my own attempts to figure out what really happened in the two disasters didn't exactly work out. So, I will ask AH.com: did the Three Mile Island meltdown have the potential to become a major nuclear disaster on the order of Chernobyl, ideally with a PoD of no more than a few days before the meltdown?
No, short of an ASB directly intervening TMI (like vanishing the containment dome and messing with the reactor's overall design and safety systems) couldn't have become a Chernobyl. Also TMI experienced a meltdown whereas Chernobyl's Reactor exploded.
First, Three Mile Island and Chernobyl were completely different reactor designs. At Three Mile Island there was a containment dome, safety systems and operators. In actuality I think only about 47 millirems were released. To give some scale to that, just being at Sea level exposes you to about 100 mrems, and Denver's ambient radiation levels are about 175 I think. So for a few minutes the area around Harrisburg PA has the ambient radiation of Colorado. It was more media storm then actual danger in my opinion. At Three mile island there was confusion over what a reading on a dial meant, which caused the problem. Basically it was a partial meltdown, caused by human error.
By comparison Chernobyl was almost the worst possible case for nuclear power plants. It was caused by negligence, stupidity and bad engineering, combined with a bad cleanup, and a collapsing Soviet Union didn't help either. It had no containment dome, it was also an A-bomb plant, the staff disabled all sorts of safety systems to test another safety system which they forgot to turn on. At Chernobyl the reactor didn't meltdown, it exploded.
Basically with NRC regulations and American safety standards, the possibility of a Chernobyl like disaster is very remote.
Given the POD of only a few days you would need ASBs to change TMI into a Chernobyl.