%%
%% The LaTeX Companion, 3ed
%%
%% Example 6-3-1 on page I-447 in "Calculating column widths".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{348.0pt}
%StartShownPreambleCommands
\usepackage{array}
\newenvironment{tabularc}[2]
 {\begin{tabular*}{#1}{*{#2}{|>{\centering\hspace{0pt}}%
      p{\dimeval{#1/(#2)-\tabcolsep*2-\arrayrulewidth*(#2+1)/(#2)}}}|}}
 {\end{tabular*}}
%StopShownPreambleCommands

\begin{document}
\begin{tabularc}{300pt}{3}    \hline
Material in column one & column two & This is column three \tabularnewline\hline
\empty
Column one  again      &
and column two         &
This is column three   \tabularnewline\hline
Once more column one   &
column two             &
Last time column three \tabularnewline\hline
\end{tabularc}
\end{document}
