\begin{frame}
  \frametitle{Example Database}
  
  \begin{exampleblock}{}
    \begin{center}
      \tableStudents\\[1ex]
      \tableExercises\\[1ex]
      \tableResults      
    \end{center}
  \end{exampleblock}

%   \begin{exampleblock}{}
%   \parbox[t]{0.55\linewidth}{
%     \tableStudents
%     
%     \medskip
%     \tableExercises
%   }\quad
%   \parbox[t]{0.35\linewidth}{%
%     \tableResults
%   }
%   \end{exampleblock}
\end{frame}