In 1996, Nintendo and HAL Labs planned to release a sequel to the quirky RPG series known as the Mother series in Japan, and the Earthbound series to the US. It was well planned out, and the project was headed by Shigesato Itoi, the same person who was involved in Mother 1 and 2. The game continued in the quirky style of the past games of the Mother series, with various different elements all combined into one game.
However, while the story was well planned storyline-wise, the development team was not used to working with 3D graphics. They had trouble developing it, and the game suffered delay after delay. It ran into further delays when, around 1998, it was announced as one of the launch titles for the 64DD, a disk-drive attachment for the Nintendo 64 which planned to enhance games. As the 64DD met it's doom in Japan, Mother 3 moved off of its list of games. Mother 3 made an appearance of Spaceworld in 1999, showing screenshots, a movie, and a playable demo, and with this, it looked as if Mother 3 would finally come out.
All these signs of completion showed nothing. The game suffered further delays following spaceworld, when finally, in late summer of 2000, it was announced that Mother 3 was cancelled. The shock hit the Earthbound community hard, but Mother 3 simply had too many problems at the time. It was full of bugs and still not completed. It was also losing interest, as the new Gamecube system made the Nintendo 64 obsolete. More information about Mother 3's cancellation can be found here.