Chapter 10 - Section 10.3 - Probability - 10.3 Assess Your Understanding - Page 705: 37

See below.

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: $X=\{BBB\}$ Hence here probability$=\frac{|X|}{|S|}=\frac{1}{8}=0.125.$