\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{A} & 1B\tnote{B} & 1C\tnote{C} \\
2A\tnote{D} & 2B\tnote{E} & 2C\tnote{F} \\
3A\tnote{G} & 3B\tnote{H} & 3C\tnote{I} \\
4A\tnote{J} & 4B\tnote{K} & 4C\tnote{L} \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]
\footnotesize
\item[A]annotation 1A
\item[B]annotation 1B
\item[C]annotation 1C
\item[D]annotation 2A
\item[E]annotation 2B
\item[F]annotation 2C
\item[G]annotation 3A
\item[H]annotation 3B
\item[I]annotation 3C
\item[J]annotation 4A
\item[K]annotation 4B
\item[L]annotation 4C
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}