\documentclass[a4paper]{article}
\usepackage{dcolumn}
\begin{document}

\begin{tabular}{lcr} \hline
1 & 2 & 3 \\ 
4 & 5 & 6 \\
7 & 8 & 9 \\ \hline
\end{tabular}

\vspace*{1cm}

\begin{tabular}{lcr} \hline
\multicolumn{2}{c}{12} & 3 \\
4 & 5 & 6 \\
7 & 8 & 9 \\ \hline
\end{tabular}

\vspace*{1cm}

\begin{tabular}{lcr} \hline
\multicolumn{2}{c}{12} & 3 \\
4 & 5 & 6 \\
\multicolumn{2}{l}{78} & 9 \\ \hline
\end{tabular}

\vspace*{1cm}

\begin{tabular}{lD{,}{.}{3}r} \hline
1 & 2,3 & 3 \\ 
4 & 5,67 & 6 \\
7 & 8,901 & 9 \\ 
0 & 1,2345 & 2 \\ \hline
\end{tabular}


\end{document}