\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{xcolor}
\usepackage{tikz}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{3}{ccc}}
\toprule
1A\tnote{1} & 1B\tnote{2} & 1C\tnote{3} \\
2A\tnote{4} & 2B\tnote{5} & 2C\tnote{6} \\
3A\tnote{7} & 3B\tnote{8} & 3C\tnote{9} \\
4A\tnote{10} & 4B\tnote{11} & 4C\tnote{12} \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]
\footnotesize
\item[1]annotation 1A
\item[2]annotation 1B
\item[3]annotation 1C
\item[4]annotation 2A
\item[5]annotation 2B
\item[6]annotation 2C
\item[7]annotation 3A
\item[8]annotation 3B
\item[9]annotation 3C
\item[10]annotation 4A
\item[11]annotation 4B
\item[12]annotation 4C
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}