Why did my cat give birth to a three-legged kitten?

Born deformed? That's something you can't help. It may be inbreeding, or it may be that the mother cat has an abnormal diet during pregnancy. There are many such examples on the Internet. But I hope you can take good care of the kitten and don't abandon it, because such a cat can't compete with the wild cats outside and will definitely die outside. Although the survival of the fittest, after all, domestic cats are breeds cultivated by human beings against nature and should be taken care of by someone.