1 and 2 are fine.
3 would be fine if it's clear that "it" refers to a cat or dog or other animal, I guess.
1. She has a brother.
2. He has a brother.
3. It has a brother.
[Can we use all the expressions? Does "It' refer to "She" or "He"? When 'It' refers to an animal, can we use Sentence 3?]
2 answers
1 and 2 are good. 3 is wrong.