Find the probability of a couple having a baby girl when their third child is? born, given that the first two children were both girls. assume boys and girls are equally likely. is the result the same as the probability of getting all girls among three children?

This is called independent probability. Just because the first two kids are girls does not change the probability of the next childs gender. 
So it is 1/2 or 50%. 
The chance of all three kids being girls is the same as tossing a coin thrice and having heads all 3 times. Thus it is 1/2 * 1/2 * 1/2 = 1/8