\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}{lcr}}
\toprule
\textbf{value1} & \textbf{group3} & \textbf{group1} \\
\textit{Sub1} & \textit{Sub2} & \textit{Sub3} \\
\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}
\end{threeparttable}
\end{table}
\end{document}