Firstly, .2 *.8 * .8 = .128, not .124

.128 is the probability that they will have: first a boy, then 2 girls.

You need to realize they are asking for a combination, not a permutation.

You can have a boy and 2 girls in three different orders: BGG, GBG, GGB. The probability you are looking for is 3*0.128= 0.384