\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{9}{lcrcrcrcr}}
\toprule
 & \textbf{group1} &  & \textbf{group1} &  & \textbf{group1} &  & \textbf{group1} &  \\
 & a &  & a &  & b &  & b &  \\
 & \textbf{group2} &  & \textbf{group2} &  & \textbf{group2} &  & \textbf{group2} &  \\
 & e &  & f &  & e &  & f &  \\
\textbf{group3} & \textbf{value1} & \textbf{mean} & \textbf{value1} & \textbf{mean} & \textbf{value1} & \textbf{mean} & \textbf{value1} & \textbf{mean} \\
\midrule
c & 1 & 1 & 2 & 2 & 5 & 5 & 6 & 6 \\
d & 3 & 3 & 4 & 4 & 7 & 7 & 8 & 8 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}