Bracketing equations across multiple lines

Question

I'm sure this one is a hoary old chestnut that's been answered before, but i can't find that answer....I have set of equations (schematically) displayed thus, using amsmath:

\begin{align*}
a &= x1  &y &=1,\\
  &= x2  &  &=2,\\
  &= x3  &  &=3.
\end{align*}

What I want to do is put the second 'column' of equations (the "y=" results) inside a single set of brackets that spans all three rows. Just can't see how...?

Answer 1

A short solution with the [left=.., right=..] keys of empheq:

\documentclass{article}
\usepackage{empheq} %% loads mathtools, hence amsmath

\begin{document}

\begin{empheq}[left ={\begin{aligned}
  a &= x1 \\ &= x2 \\ &= x3
\end{aligned}\qquad\empheqlbrace}, right =\empheqrbrace]{gather*}
\begin{aligned}
y &=1,\\
 &=2,\\
 &=3.
 \end{aligned}
\end{empheq}

\end{document} 

Answer 2

Something like this?

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\left.
\begin{aligned}
a   &=  x_1 \\
    &=  x_2 \\
    &=  x_3
\end{aligned}
\right\}
\quad
\left\{
\begin{aligned}
y   &=  1 \\
    &=  2 \\
    &=  3
\end{aligned}
\right\}
\end{equation*}

\end{document}

EDIT

Here are some other approaches to this. The second one needs \usepackage{blkarray} in your preamble

\begin{equation*}
\left.
\begin{array}{r}
a   =   x_1 \\
    =   x_2 \\
    =   x_3
\end{array}
\right\}
\quad
\left\{
\begin{array}{r}
y   =  1 \\
    =  2 \\
    =  3
\end{array}
\right\}
\end{equation*}

\bigskip

\begin{center}
\begin{blockarray}{r\}c\{r\}}
$a  =   x_1$ & \quad & $y   =   1$ \\
$   =   x_2$ & \quad & $    =   2$ \\
$   =   x_3$ & \quad & $    =   3$
\end{blockarray}

Answer 3

Try this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\left.
\begin{aligned}
a &= x1  &&&y &=1,\\
  &= x2  &&&  &=2,\\
  &= x3  &&&  &=3.
\end{aligned}
\right\}
\end{equation*}
\end{document}

Maybe this:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\begin{aligned}
a &= x1\\
  &= x2\\
  &= x3
\end{aligned}
\quad\left\{
\begin{aligned}
y &=1,\\
  &=2,\\
  &=3.
\end{aligned}
\right\}
\end{equation*}
\end{document}