according to the lenient opinion of the Chazon Ish, why is the age for yichud 3 whilst for ervah etc it depends on appearance?
The halacha of yichud is a concern for forbidden relations. This applies to any girl who can have what is considered relations according to the Torah. This is from the age of 3 years old. While it seems to us far fetched, the prohibition of yichud is a Torah prohibition which means the Torah established that there is some concern for this possible aveirah, and thus the yichud is forbidden.
The question of Ervah is essentially whether her body has the form of a woman which could cause improper thoughts to one who sees it. This, argues the Chazon Ish only happens when her body begins developing as that of a woman’s.