Style de numérotation différent dans la table des matières de celles des sections

 \documentclass{beamer}

\usetheme[progressbar=frametitle]{metropolis}
\setbeamertemplate{frame numbering}[fraction]
\useoutertheme{metropolis}
\useinnertheme{metropolis}
%\usefonttheme{Berlin}
\usecolortheme{orchid}
\setbeamercolor{background canvas}{bg=white}

\renewcommand{\thesection}{\Roman{section}.}
\renewcommand{\thesubsection}{\arabic{subsection}}
\renewcommand{\thesubsubsection}{\alpha{subsubsection}}

\usepackage{romannum}

\makeatletter

\setbeamertemplate{section in toc}{%
  \leavevmode%
  % prevents the period to be printed with the first/last section option
  \ifnum\beamer@tempcount>\beamer@toclastsection
  \else
  \ifnum\beamer@tempcount>0 
    \Romannum{\inserttocsectionnumber}
  \fi\fi%
  \inserttocsection\par%
}

\setbeamertemplate{subsection in toc}{\leavevmode\leftskip=2em\rlap{\hskip-2em\Romannum{\inserttocsectionnumber}.\inserttocsubsectionnumber}\inserttocsubsection\par}

\setbeamertemplate{subsubsection in toc}
{\leavevmode\leftskip=3em%
 \rlap{\hskip-3em\Romannum{\inserttocsectionnumber}.\inserttocsubsectionnumber.\@alph{\inserttocsubsubsectionnumber}}%
 \inserttocsubsubsection\par}

\makeatother

\makeatletter
\setbeamertemplate{section page}{%
  \centering
  \begin{minipage}{22em}
    \raggedright
    \usebeamercolor[fg]{section title}
    \usebeamerfont{section title}
    \thesection~\insertsectionhead\\[-1ex]
    \usebeamertemplate*{progress bar in section page}
    \par
    \ifx\insertsubsectionhead\@empty\else%
      \usebeamercolor[fg]{subsection title}%
      \usebeamerfont{subsection title}%
      \insertsubsectionhead
    \fi
  \end{minipage}
  \par
  \vspace{\baselineskip}
}
\makeatother

\begin{document}
\begin{frame}{Titre}
\tableofcontents
\end{frame}
\section{Rappels}
\subsection{Fondements}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsubsection{Hypoth\`eses}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsubsection{Causes}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsection{Implications}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsection{Critiques}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\section{Th\'eorie}
\subsection{Plan}
\begin{frame}{Titre}
Bla bla.
\end{frame}
\subsection{\'Equilibre}
\begin{frame}{Titre}
Bla bla.
\end{frame}

\end{document}