Es gilt
$$\begin{pmatrix} n\\k \end{pmatrix} = \begin{pmatrix} n\\n-k \end{pmatrix}$$
Also
$$\begin{pmatrix} 3n\\3n-3 \end{pmatrix} = \begin{pmatrix} 3n\\3n - (3n - 3) \end{pmatrix} = \begin{pmatrix} 3n\\3 \end{pmatrix} = \frac{3·n·(3·n - 1)·(3·n - 2)}{6}$$