多列间距不相等

Question

使用多列时，列之间的间距不一样。（第 2 列距离第 1 列比距离第 3 列更近。）我怎样才能使它们相同？

\documentclass{article}
\begin{document}
\begin{tabular}{ccc|c}
\hline
\multicolumn{3}{c|}{Here is a long text} & P \\
 1 & 2 & 3 & $A$ \\
 2 & 1 & 3 & $B$ \\
 3 & 1 & 2 & $C$ \\
 3 & 2 & 1 & $D$ \\
\hline
\end{tabular}
\bigskip
\begin{tabular}{ccc|c}
\hline
 & & & P \\
 1 & 2 & 3 & $A$ \\
 2 & 1 & 3 & $B$ \\
 3 & 1 & 2 & $C$ \\
 3 & 2 & 1 & $D$ \\
\hline
\end{tabular}
\end{document}

Answer 1

我的评论的概念证明答案：

\documentclass{article}

\usepackage{array}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\newlength\mylen
\newcommand\seteqwidth[3]
  {%
    \settowidth#1{#2}%
    \setlength#1{\dimexpr(#1-(#3\tabcolsep-\tabcolsep)*2)/#3}%
  }

\begin{document}
\begingroup
\seteqwidth\mylen{Here is a long text}{3}
\begin{tabular}{*3{C{\mylen}}|c}
\hline
\multicolumn{3}{c|}{Here is a long text} & P \\
 1 & 2 & 3 & $A$ \\
 2 & 1 & 3 & $B$ \\
 3 & 1 & 2 & $C$ \\
 3 & 2 & 1 & $D$ \\
\hline
\end{tabular}
\endgroup

\bigskip
\begin{tabular}{ccc|c}
\hline
 & & & P \\
 1 & 2 & 3 & $A$ \\
 2 & 1 & 3 & $B$ \\
 3 & 1 & 2 & $C$ \\
 3 & 2 & 1 & $D$ \\
\hline
\end{tabular}
\end{document}