Separate subsubsections by commas in beamer table of contents

I am trying to create a table of content in beamer presentations where only the subsubsections are shown separated by commas but I cannot find a way to achieve this.

documentclass{beamer}

begin{document}

begin{frame}{Content}{}
tableofcontents
end{frame}

%---------------------------    
section{Section 1}
subsection{Subsection 1.1}
subsubsection{Subsubsection 1.1.1}

begin{frame}{Section 1}{Subsection 1.1}    
Subsubsection 1.1.1    
end{frame}    
%---------------------------------------    
subsubsection{Subsubsection 1.1.2}    
begin{frame}{Section 1}{Subsection 1.1}   
Subsubsection 1.1.2    
end{frame}    
%---------------------------------------    
subsubsection{Subsubsection 1.1.3}   
begin{frame}{Section 1}{Subsection 1.1}    
Subsubsection 1.1.3    
end{frame}

end{document}

What I need to produce is:

Section 1    
  Subsection 1.1    
    Subsubsection 1.1.1, Subsubsection 1.1.2, Subsubsection 1.1.3

instead of:

Section 1    
  Subsection 1.1    
    Subsubsection 1.1.1    
    Subsubsection 1.1.2    
    Subsubsection 1.1.3

Many thanks for any suggestions!

setbeamertemplate{subsection in toc}{parhspace*{2em}~inserttocsubsectionpar}
setbeamertemplate{subsubsection in toc}{hspace*{2.5em}{~inserttocsubsubsection}}