\documentclass{article} 
\usepackage[french]{babel}
\usepackage[utf8]{inputenc} 
\usepackage{lipsum}
\usepackage[a4paper]{geometry}

\usepackage{titlesec}

\titleformat{\section}[block]
{\normalfont\bfseries}{\Large\fbox{\begin{minipage}[c][2em][c]{2em}\centering\itshape\thesection\end{minipage}}}{2em}{\LARGE}
\titleformat{\subsection}[block]
{\normalfont\bfseries}{\Large\fbox{\begin{minipage}[c][2em][c]{2em}\centering\itshape\thesection\end{minipage}}.\arabic{subsection}}{1.5em}{\Large}
\titleformat{\subsubsection}[block]
{\normalfont\bfseries}{\Large\fbox{\begin{minipage}[c][2em][c]{2em}\centering\itshape\thesection\end{minipage}}.\arabic{subsection}.\arabic{subsubsection}}{1em}{\large}

% Fin du préambule.

\begin{document} 
	
	\section{Une grosse section}
	\subsection{Une première partie}
	\subsubsection{Petitun}
	\subsubsection{Petideux}
	\subsection{Une autre}
	\subsection*{Et une derinère sans numéro}
	\section{Grand deux !}
	
	\paragraph{Un titre de paragraphe} Lorem ipsum dolor sit amet, consectetuer adipscing elit.
\end{document}