The Taylor series expansion of $e^x$ about $x=0$ is $$e^x = \sum\limits_{n=0}^{\infty} \frac {x^n} {n!},\ \text {for all }\ x \in \Bbb R.$$
Observe that your sum can be written as $$\sum\limits_{n=0}^{\infty} \frac {3^n} {n!} = e^3.$$
The Taylor series expansion of $e^x$ about $x=0$ is $$e^x = \sum\limits_{n=0}^{\infty} \frac {x^n} {n!},\ \text {for all }\ x \in \Bbb R.$$
Observe that your sum can be written as $$\sum\limits_{n=0}^{\infty} \frac {3^n} {n!} = e^3.$$