\begin{block}{Graphes des itérations de  
$(x_1, x_2, x_3) \mapsto 
((\overline{x_1} + \overline{x_2}).x_3,
x_1.x_3,
x_1 + x_2 + x_3)$.}
\vspace{-2em}
\begin{figure}[ht]
  \begin{center}
    \subfloat[$\textsc{gis}(f)$]{
      \begin{minipage}{0.33\textwidth}
        \begin{center}
          \includegraphics[scale=0.25]{fsig2}
        \end{center}
      \end{minipage}
      \label{fig:fsig}
    }
    \subfloat[$\textsc{giu}(f)$]{
      \begin{minipage}{0.33\textwidth}
        \begin{center}
          \includegraphics[scale=0.25]{faig2}
        \end{center}
      \end{minipage}
      \label{fig:faig}
    }   
    \subfloat[$\textsc{gig}(f)$]{
      \begin{minipage}{0.33\textwidth}
        \begin{center}
          \includegraphics[scale=0.25]{fgig2}
        \end{center}
      \end{minipage}
      \label{fig:fgig}
    }
  \end{center}
  %\caption{}
\end{figure}
\end{block}
% On remarque le cycle $((101,111),(111,011),(011,101))$ 
%  à la \textsc{Figure}~(\ref{fig:fsig}).