multi aligned cases environ template

ltxtest.tex

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\MHInternalSyntaxOn
\def\MT_start_cases:nnnnn #1#2#3#4#5{%
  \RIfM@\else
   \nonmatherr@{\begin{\@currenvir}}
  \fi
  \MH_group_align_safe_begin:
  \left#5
  \vcenter \bgroup
     \Let@ \chardef\dspbrk@context\@ne \restore@math@cr
     \let  \math@cr@@\AMS@math@cr@@
     \spread@equation
     \ialign\bgroup
       \strut@#2 &#1\strut@
       #3 &#1\strut@
       #4
       \crcr
}
\def\MH_end_cases:{\crcr\egroup
 \restorecolumn@
 \egroup
 \MH_group_align_safe_end:
}
\newcommand*\mynewcases[7]{%
 \newenvironment{#1}
   {\MT_start_cases:nnnnn {#2}{#3}{#4}{#5}{#6}}
   {\MH_end_cases:\right#7}
}

\mynewcases{mydcases}{\relax}
  {$\m@th{##}$\hfil}{$\m@th{##}$\hfil}{$\m@th{##}$\hfil}
  {.}{\rbrace}
\MHInternalSyntaxOff

\begin{document}
\[
    \begin{mydcases}
      a &{} = b &{} = c \\
      a &{} = b &{} = c \\
      a &{} = b &{} = c
    \end{mydcases}
\]
\end{document}