Three-Mile Island is the largest of the Nowhere Islands and located slightly farther away from the others. During the war that left almost no human survivors save for the ones that would find Nova Island, this island was the only one of the Nowhere Islands which was bombed. It was once a great military base, but now Three-Mile Island is a sprawling pile of trash, sludge and radiation. Even the sky around Three-Mile Island is always a hazy green/blue when it isn't grey with smog. At the end of the game, the Pig Army transports all villagers from the other islands to New Pork City built above this island. After completing the Empire Porky Building dungeon, the elevator shaft leads to the actual island. It may take a long time to ever reach the Final Cave of the Future buried far beneath the destruction and pollution- this utopia of trash is swarming with nuclear-powered foes.
The name "Three-Mile Island" is inspired by the real-life Three-Mile Island laced with radiation where animals developed supposed mutations. In Mother 3.5, Three-Mile Island is in fact the largest island of the Nowhere Islands, at three square miles in size.