\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{I} & 1B\tnote{II} & 1C\tnote{III} \\
2A\tnote{IV} & 2B\tnote{V} & 2C\tnote{VI} \\
3A\tnote{VII} & 3B\tnote{VIII} & 3C\tnote{IX} \\
4A\tnote{X} & 4B\tnote{XI} & 4C\tnote{XII} \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]
\footnotesize
\item[I]annotation 1A
\item[II]annotation 1B
\item[III]annotation 1C
\item[IV]annotation 2A
\item[V]annotation 2B
\item[VI]annotation 2C
\item[VII]annotation 3A
\item[VIII]annotation 3B
\item[IX]annotation 3C
\item[X]annotation 4A
\item[XI]annotation 4B
\item[XII]annotation 4C
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}