It is not possible to answer the question for a number of reasons.
The gender of children are not independent of one another. They depends on the parents.
Second, the feeble-mindedness of the child or otherwise is greatly influenced by their nurture: the family and school play an important role in that context. This will affect feeble-minded boys equally.
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The probability of a boy is still 0.5 no matter how many prior children there are.
3 out of 7
There is no simple answer to the question because the children's genders are not independent events. They depend on the parents' ages and their genes. However, if you assume that they are independent events then, given that the probability of a boy is approx 0.52, the probability of the other two being boys is 0.2672.
The conditional probability is 1/4.
Assuming it is a fair coin, the probability is 1/24 = 1/16.