{ \newcommand{\nodea}{\node[draw,circle] (a) {$3$}
;}\newcommand{\nodeb}{\node[draw,circle] (b) {}
;}\newcommand{\nodec}{\node[draw,circle] (c) {$1, 2$}
;}\begin{tikzpicture}[auto]
\matrix[column sep=.3cm, row sep=.3cm,ampersand replacement=\&]{
         \& \nodea  \&         \&         \\ 
         \&         \&         \& \nodeb  \\ 
         \&         \& \nodec  \&         \\
};

\path[ultra thick, red] (b) edge (c)
	(a) edge (b);
\end{tikzpicture}}