How to add a left-hand brace to this system of equations?

Question

How to

My code

\documentclass[12pt,a4paper]{article}
\usepackage{amsmath}
\begin{document}
    \begin{alignat}{3}
        a_1x &+ b_1y &&+ c_1z &&= d_1 \label{eq1}\\
        a_2x &+ b_2y &&+ c_2z &&= d_2 \label{eq2}\\
        a_3x &+ b_3y &&+ c_3z &&= d_3 \label{eq3}
    \end{alignat}
    From the equation \eqref{eq1} we have
    \end{document}

Answer 1

The following solution employs the empheq package and its eponymous environment.

Note that it's not necessary to employ {alignat}{3} for the equations at hand -- align will do just fine.

\documentclass[12pt,a4paper]{article}
\usepackage{empheq} % 'empheq' loads 'amsmath' automatically

\begin{document}
\begin{empheq}[left=\empheqlbrace]{align}
   a_1x + b_1y + c_1z &= d_1 \label{eq1} \\
   a_2x + b_2y + c_2z &= d_2 \label{eq2} \\
   a_3x + b_3y + c_3z &= d_3 \label{eq3}
\end{empheq}
From equation \eqref{eq1} we have \dots
\end{document}

Answer 2

With {DispWithArrows} of witharrows.

\documentclass[12pt,a4paper]{article}
\usepackage{witharrows}
\usepackage{amsmath}

\begin{document}
\begin{DispWithArrows}<>
   a_1x + b_1y + c_1z &= d_1 \label{eq1} \\
   a_2x + b_2y + c_2z &= d_2 \label{eq2} \\
   a_3x + b_3y + c_3z &= d_3 \label{eq3}
\end{DispWithArrows}
From equation \eqref{eq1} we have \dots
\end{document}