\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}{lcc}}
\toprule
 & \multicolumn{2}{c}{\textbf{Value B}} \\
\cmidrule{2-3}
 & a & b \\
\textbf{Custom C} & \multicolumn{2}{c}{\textbf{Custom A}} \\
\midrule
x & 1 & 2 \\
y & 3 & 4 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}