assuming boys and girls are equally likely find the probability of a couple having a baby boy when their third child is born. given the first two children were both boys

After the first 2 children are born, whatever their sex, the chance of having either a boy or a girl is 1/2 or %50. (Independent event)

Prior to having any children the chance of having 3 children that are girls is 1/2 x 1/2 x 1/2 = 1/8 or %12.5 (Dependent event)