\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}{llc}}
\toprule
\textbf{Group 1} &  & \textbf{value1} \\
\midrule
a & \textbf{mean} & 2.5 \\
b & \textbf{mean} & 6.5 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}