- "Six months ago, I designed a never-seen operation not separating anymore but creating. I transformed my three Rottweilers into a beautiful three-hound construction."
- —Josef Heiter[src]
In The Human Centipede (First Sequence), the 3Dog was an experiment created by Josef Heiter as a rough idea for his 'human centipede' creation and consisted of three of his pet rottweiler being joint mouth to rectum.
Before the events of The Human Centipede (First Sequence), reputable surgeon Josef Heiter came up with the idea that, instead of separating Siamese twins, he would do the opposite by surgically connecting multiple people. He began the experiment on his three pet rottweilers, joining them rectum to mouth. One of the dogs had attempted to escape, which led to Heiter putting it in the middle of the other two dogs as punishment. By becoming the "middle piece", the dog would suffer through twice the surgical healing pain, something Heiter would reflect on with a human victim in the future.
The dogs had died sometime after the operation, although Heiter had still managed to get photographs to keep as 'memories' of his work. It's not clear how long the dogs lived for. After their deaths, he buried them in the garden with a gravestone.
When victim Lindsay attempted to escape after being kidnapped by Heiter, Heiter reflected on how one of his dogs tried to escape, and how he punished it by connecting it in between the other two dogs during the 3Dog surgery. He then decided to place Lindsay in the middle of his human centipede as punishment.
Heiter also kept a framed photo by the side of his bed of his 3Dog.
- Tom Six revealed in the audio commentary for First Sequence that the 3Dog photos were created with Photoshop.