\documentclass[border=2pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \node[circle, draw] (1) at (0,0) {1};
  \node[circle, draw] (2) at (2,0) {2};
  \node[circle, draw] (3) at (1,1.5) {3};
  \draw (1) -- (2);
  \draw (2) -- (3);
  \draw (3) -- (1);
\end{tikzpicture}
\end{document}
