Answer
See below.
Work Step by Step
We know that probability$=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}.$
Let $B$ represent a boy, $G$ a girl, then the sample space is: $X=\{BBB,BBG,BGB,GBB,GGB,GBG,BGG,GGG\}$.
The good outcomes are: $X=\{GGG\}$
Hence here probability$=\frac{|X|}{|S|}=\frac{1}{8}=0.125.$