\documentclass{article}
\usepackage{array}
\usepackage{pstricks}
\usepackage{calc}



\newlength{\entwd}
\newlength{\entht}
\newlength{\cellwd}
\newlength{\cellht}
 

\newcommand{\drawx}[1]{%
  \settowidth{\entwd}{#1}%
  \settoheight{\entht}{#1}%
  \setlength{\cellwd}{\entwd+\tabcolsep}%
  \setlength{\cellht}{\entht+0.5\jot}%
  \psline(-\tabcolsep,-\jot)(\cellwd,\cellht)%
  \psline(\cellwd,-\jot)(-\tabcolsep,\cellht)}


\begin{document}


\begin{tabular}{|l|l|l|}
  \hline
  field 1 & field 2 & \drawx{field 3} field 3\\ 
  \hline
\end{tabular}


\vspace{2cm}


\begin{tabular}{|l|l|l|}
  \hline
  field 11 & field 12 & \drawx{field 13} field 13\\ 
  \hline
  field 21 & \drawx{field 22} field 22 & field 23\\ 
  \hline
\end{tabular}


\end{document}