\documentclass{article} \renewcommand*{\thesection}{\Roman{section}.} \renewcommand*{\thesubsection}{\arabic{subsection}.} \renewcommand*{\thesubsubsection}{\roman{subsubsection}.} \begin{document} \section{Section} \subsection{Subsection} \subsection{Subsection} \subsubsection{Subsubsection} \subsubsection{subsubsection} \end{document}