$$P\left(A\right)=\frac{\left|A\right|}{\left|\Omega \right|}$$

|Ω|-Total number of outcomes
From 5 numbers 3 need to bee selected. This is a combination without repetitionf of 5 elements of third order!

$$$$$$\left|\Omega \right|=C_3^5=\left(\begin{array}{c}5\\ 3\end{array}\right)$$

$$$$$$\left|\Omega \right|=\left(\begin{array}{c}5\\ 3\end{array}\right)=\frac{5·4·3}{3!}=\frac{5·4·3}{3·2·1}=5·2$$

|A|-Number of ways an event can happen