\documentclass{article}
\usepackage{dcolumn}

\begin{document}

\subsection*{Dancing Decimals}

\begin{tabular}{lcr}
left col & center col & right col \\
 1 &  -0.120  &  0.12  \\
 2 & 0.254 & 0.254 \\
 3 &  1.456 &  -1.234 \\
\end{tabular}

\subsection*{Dcolumn, first try}

\begin{tabular}{lD{.}{.}{2}r}
left col & center col & right col \\
 1  &  -0.120  &  0.12  \\
 2  & 0.254 & 0.254 \\
 3  &  1.456 &  -1.234 \\
\end{tabular}

\subsection*{Dcolumn, centered multicolumn}

\begin{tabular}{lD{.}{.}{2}r}
left col & \multicolumn{1}{c}{center col} & right col \\
 1  &  -0.120  &  0.12  \\
 2  & 0.254 & 0.254 \\
 3  &  1.456 &  -1.234 \\
\end{tabular}

\end{document}