\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\usepackage{xcolor}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{3}{ccc}}
\toprule
\textbf{value1} & \textbf{Group 3} & \textbf{Group 1\tnote{1}} \\
\midrule
1 & c & a \\
2 & c & a \\
3 & d & a \\
4 & d & a \\
5 & c & b \\
6 & c & b \\
7 & d & b \\
8 & d & b \\
\bottomrule
\end{tabular}
\begin{tablenotes}[flushleft]
\footnotesize
\item[1]is annotated
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}