\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{0mm}

\usepackage{amsmath,array}
\renewcommand\arraycolsep{4pt} % default value: 6pt


\begin{document}
\begin{preview}
\begin{equation*}
S^T \cdot A \cdot S =
\left( \,
\begin{array}{r@{}r@{}r r r}  % @{} is used twice to suppress intercolumn whitespace
  \underbrace{
      \begin{array}{rrr}    % First block (1)
        1 &        &   \\
          & \ddots &   \\
          &        & 1 \\
      \end{array}
  }_{r_+(s)-mal}\\
  & 
  \underbrace{
      \begin{array}{rrr}    % Second block (-1)
        -1 &        &   \\
           & \ddots &   \\
           &        & -1\\
      \end{array}
  }_{r_{-}(s)-mal} \\
  & & 
  \underbrace{
      \begin{array}{rrr}    % Third block
        0  &        &  \\
           & \ddots &  \\
           &        & 0\\
      \end{array}
  }_{r_0(s)-mal} \\
\end{array}\,\right)
\end{equation*}
\end{preview}
\end{document}