[latex] latex tabular width the same as the textwidth

The tabularx package gives you

  1. the total width as a first parameter, and
  2. a new column type X, all X columns will grow to fill up the total width.

For your example:

\usepackage{tabularx}
% ...    
\begin{document}
% ...

\begin{tabularx}{\textwidth}{|X|X|X|}
\hline
Input & Output& Action return \\
\hline
\hline
DNF &  simulation & jsp\\
\hline
\end{tabularx}