We can write this as:

(1/2)^3= 1/8

There is a 1/8 probability that they will have exactly one girl.

this really requires additional information because there may be studies that support greater than a 50-50 chance of a boy or girl in any given family circumstance

we will, however, assume that there is a 50-50 chance of any family having a boy or a girl and we will assert that each birth is completely random, therefore the probability that any child will be a girl is 50-50

three children, each a separate and random event and the family has girl-boy-boy or boy-girl-boy or boy-boy-girl, three possibilities:

(1/2)*(1/2)*(1/2) * 3 = 3/8

there is a 3-in-8 chance of having only one girl