TeX source:

\begin{align*} \mathrm{p}=\frac{\mathrm{n} !}{\left(\mathrm{n}_{1} !\right)\left(\mathrm{n}_{2} !\right)\left(\mathrm{n}_{3} !\right)} \mathrm{p}_{1}^{\mathrm{n}_{1}} \mathrm{p}_{2}^{\mathrm{n}_{2}} \mathrm{p}_{3}^{\mathrm{n}_{3}} \end{align*}