Is it permissible for a third-gender person to fast? And is their repentance permissible?

If by "third gender" the ambiguous intersex individual is meant, then they have specific rulings: men should treat them as a woman, and women as a man. They are obligated to fulfill religious duties, while observing the most cautious approach in their acts of worship and dealings. However, if what is meant are men who imitate women, then these are cursed, and they must repent, adhere to religious obligations, and avoid prohibited acts.