How to put a brace on the right, not left, to group cases?

With the help of the aligned environment from amsmath, you can define your own environment:

\documentclass{article}
\usepackage{amsmath}

\newenvironment{rcases}
  {\left.\begin{aligned}}
  {\end{aligned}\right\rbrace}

\begin{document}

\begin{equation*}
\begin{rcases}
  a &= b + c \\
  d &= e + f + g \\
  h &= i + j + k + l
\end{rcases}
\text{ three equations}
\end{equation*}

\end{document} 

use package mathtools. There is also a drcases environment

\documentclass{article}
\usepackage{mathtools}
\begin{document}

\[
\begin{rcases*}
E = m c^2 & foo \\
\int x-3\, dx & barbaz
\end{rcases*} y=f(x)
\]

\end{document}

Maybe something like this:

\documentclass{article}
\usepackage{amsmath}

\begin{document}
\begin{equation}
  \left.\begin{aligned}
  a&=bbb\\
  c&=ddddddd\\
  e&=ffffff
\end{aligned}\right\} = stuff
\end{equation}
\end{document}