\documentclass[10pt,a4paper]{article}
\usepackage[left=2cm,right=2cm,top=2cm,bottom=2cm]{geometry}
\usepackage{multirow}
\begin{document}
\begin{center}
\begin{Large}
\textbf{Table Demo}
\end{Large}
\end{center}
\section*{Marks Table}
\begin{tabular}{|c|c|c|c|c|c|}
\hline
Sl.No & USN & Student Name & \multicolumn{3}{c|}{Marks}\\
\cline{4-6}
& & & Subject1 & Subject2 & Subject3 \\ \hline
1 & 1IC24CS051 & Sourabh & 89 & 60 & 90 \\ \hline
2 & 1IC24CS056 & Varun & 78 & 45 & 98 \\ \hline
3 & 1IC24CS053 & THARUN & 67 & 55 & 59 \\ \hline
\end{tabular}
\end{document}