\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{4}{lccc}}
\toprule
 & \multicolumn{3}{c}{\textbf{B}} \\
\cmidrule{2-4}
 & 2 & 4 & 8 \\
\textbf{A} & \multicolumn{3}{c}{\textbf{value}} \\
\midrule
'a' &  & 6 &  \\
'b' & 4 & 5 &  \\
'c' & 2 & 1 & 3 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}