\documentclass{article}


\usepackage{amsmath}

\begin{document}

$\begin{matrix}
  1&2&3&\hdotsfor{2}&n\\
  2&3&\hdotsfor{2}&n&n+1\\
  3&\hdotsfor{2}&n&n+1&n+2
\end{matrix}$

\end{document}
