\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{2}{cc}}
\toprule
A\tnote{1} & B\tnote{2} \\
C\tnote{3} & D\tnote{1} \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft,para]
\footnotesize
\item[1]Note 1
\item[2]Note 2
\item[3]Note 3
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}