\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{7}{lccrccr}}
\toprule
 & \multicolumn{2}{c}{\textbf{group1}} &  & \multicolumn{2}{c}{\textbf{group1}} &  \\
\cmidrule{2-3}\cmidrule{5-6}
 & \multicolumn{2}{c}{a} &  & \multicolumn{2}{c}{b} &  \\
 & \multicolumn{2}{c}{\textbf{group2}} &  & \multicolumn{2}{c}{\textbf{group2}} &  \\
\cmidrule{2-3}\cmidrule{5-6}
 & e & f &  & e & f &  \\
\textbf{group3} & \multicolumn{2}{c}{\textbf{value1}} & \textbf{mean} & \multicolumn{2}{c}{\textbf{value1}} & \textbf{mean} \\
\midrule
c & 1 & 2 & 1.5 & 5 & 6 & 5.5 \\
d & 3 & 4 & 3.5 & 7 & 8 & 7.5 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}