tripedality
noun
Plural: tripedalities
The quality of having three legs; three-leggedness; tripedism.
Examples:
So I must not withhold that over time we have gathered a number of supporting clues on the subject of tripedality. (Context: Dr∙ Barton's group has discovered a three-legged human skeleton, which initiated decades-long research. Source: J∙ Okram - The Mystery of the Rammed Key.)