Align tabular cells at plus or minus signs

Removing the intercolumn space and using math mode

\documentclass{article}
\usepackage{array,booktabs}
\begin{document}
\begin{table}
\begin{tabular}{l*{3}{>{$}r<{$}@{}>{$}l<{$}}>{$}l<{$}}
    \toprule
    Examples & \multicolumn{2}{l}{value1} & \multicolumn{2}{l}{value2} & \multicolumn{3}{l}{value3 (+smth)} \\
    example1 &   1 & -1                   &  12 & +?                   &  12 & +? & (+123)                  \\
    example2 &  12 & +5                   & 123 & +?                   & 123 & +? & (+12)                   \\
    example3 & 123 & -10                  &   1 & +?                   &   1 & +? & (+1234)                 \\
    example4 &  23 & +123                 & 234 & +?                   & 234 & +? & (+23)                   \\
    example5 & 234 & +4                   &  23 & +?                   &  23 & +? & (+234)                  \\
    \bottomrule
\end{tabular}
\end{table}
\end{document}

You have three composite columns with the following pattern: number ± number, this can be set up as @{}>{${}}c<{{}$}@{}. A new column type can be handy for this, see below. I would also combine the last column with the previous one to be similar to their header.

\documentclass{article}
\usepackage{array,booktabs}
\begin{document}

\begin{table}
\newcolumntype{C}{ @{}>{${}}c<{{}$}@{} }
\begin{tabular}{l *3{rCl} }
    \toprule
    Examples & \multicolumn{3}{c}{value1} & \multicolumn{3}{c}{value2} & \multicolumn{4}{c}{value3 (+smth)} \\
    example1 & 1   & - & 1                & 12  & + & ?                & 12  & + & ? (+123)               \\
    example2 & 12  & + & 5                & 123 & + & ?                & 123 & + & ? (+12)                \\
    example3 & 123 & - & 10               & 1   & + & ?                & 1   & + & ? (+1234)              \\
    example4 & 23  & + & 123              & 234 & + & ?                & 234 & + & ? (+23)                \\
    example5 & 234 & + & 4                & 23  & + & ?                & 23  & + & ? (+234)               \\ \bottomrule
\end{tabular}
\end{table}

\end{document}