Answer
$0.125$ or $\dfrac{1}{8}$
Work Step by Step
We know that probability$=\frac{\text{number of favourable outcomes}}{\text{number of all outcomes}}.$
The possible outcomes: $\{BBB,GBB,BGB,BBG,BGG,GBG,GGB,GGG\}$ where B represents a boy and G represents a girl.
Since there is $1$ possible outcome with $3$ boys and $8$ outcomes in total, then
$P(\text{3 boys})=\dfrac{1}{8}=0.125$
