With 3 children, you have 8 possibilities:
BBB, BBG, BGB, BGG, GBB, GBG, GGB, GGG
Probability (3rd child is a girl | 2 are boys) = Probability(3rd child is a girl AND 2 are boys)/Probability (First 2 are boys).
Probability (3rd child is a girl AND First two are boys) = 3/8
Probability...