\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}