Please help me understand conditional probability.

Q: Find the probability of a couple having a baby girl when their fourth child is born, given that the first 3 children are girls.

A: I think the answer is 0.5. Because regardless of the situation the probability will always be 0.5 as each event is independent.

How can I solve this problem in terms of P(B|A)=P(A and B)/P(A) (Headbang)