Centering of content below cmidrule

I'm trying to figure out how I can center the columns below a multicolumn statement with a very long text because my current result does not look fine.

I didn't find anything related but it is also very hard to describe by a term so maybe I just did not have the right term.

\documentclass[a4paper, 12pt]{article}
\usepackage[table]{xcolor}
\usepackage{longtable}
\usepackage{booktabs}

\ra{1.3}
\begin{document}
\begin{longtable}{>{\kern-\tabcolsep}lll<{\kern-\tabcolsep}}
\toprule
\multicolumn{3}{l}{Long Text  .....}\\
\cmidrule{1-3}
A & B & C \\
\midrule
A & B & C \\
A & B & C \\
\end{longtable}
\end{document}


• How exactly do you want it to look?
– cfr
Sep 13, 2015 at 2:16
• The A, B and C should be centered below the long text and not somehow left aligned as there
– flp
Sep 13, 2015 at 2:19
• The negative \kern is shifting the columns left. If you want that, then I don't see how they can be centred.
– cfr
Sep 13, 2015 at 2:20
• Actually, I also had the problem before with the @{} instead of the kerns
– flp
Sep 13, 2015 at 2:21

Something like this? Note that the kerns won't make any difference here, because they are not implemented for the longest row which obviously determines the total width.

\documentclass[a4paper, 12pt]{article}
\usepackage{longtable,array,calc}
\usepackage{booktabs}
\ra{1.3}
\begin{document}
\newcolumntype{C}{>{\centering\arraybackslash}p{\shorttextlength}}
\newlength\longtextlength
\newlength\shorttextlength
\settowidth\longtextlength{Long Text \dots.}