Connotation of "cancer" for native speaker
The German word krebs can refer to the animals crabs or the disease cancer. In English the word crab refers to just the animal. The word cancer refers in normal usage mainly to the disease, but sometimes to the astrological sign. The word cancer cannot be used to refer to the animal in every day usage.
For these reasons, The Three Little Crabs would be a suitable title, but the version with the disease would clearly not.