\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}