Translation:They have three younger brothers.
Does that make sense? They're using the male 他 form here, so if they're brothers and have "three younger brothers", wouldn't at least one of them have at least four younger brothers?
Maybe they're a brother and sister (I think that would still use 他?) where the brother is the eldest, and they also have three more brothers who are younger?
This wouldn't be used in English. 'Younger' is a comparative indicating the brothers' ages are less than the person/people in question.