\documentclass[12pt]{article}
\pagestyle{empty}
\usepackage{tabularx}

\newcolumntype{C}{>{\centering}X}
\begin{document}

\noindent%
\begin {tabularx}{\linewidth}{|*{3}{C|}}\hline
1\\blah & 2 & 345\\blah \tabularnewline\hline
\end {tabularx}

\end{document}