Manual Tag without amsmath

Question

Hi I am writing in an iopart document and hence, frustratingly, amsmath doesn't seem like an option. Nonetheless I have three equations on a single line like this:

\begin{equation}
some equation, \qquad 
another equation, \qquad 
third equation,
\end{equation}

and I wish for the equation number to read (4,5,6) and not just (4) as it does at the moment. I would use the tag command and just manually tag it but the tag command does not work because of amsmath not being available. Any help would be greatly appreciated. Thank you

Answer 1

For a one-off case, you can do it manually:

\documentclass[12pt]{iopart}

\makeatletter
\newcommand{\advancethree}[3]{%
  \stepcounter{equation}%
  \edef\@currentlabel{\theequation}\label{#1}%
  \edef\temp{\theequation, }%
  \stepcounter{equation}%
  \edef\@currentlabel{\theequation}\label{#2}%
  \edef\temp{\temp\theequation, }%
  \stepcounter{equation}%
  \edef\@currentlabel{\theequation}\label{#3}%
  \edef\temp{\temp\theequation}%
  \renewcommand{\theequation}{\temp}%
}
\let\ORItheequation\theequation
\newcommand{\restore}{\addtocounter{equation}{-1}\let\theequation\ORItheequation}
\makeatother

\begin{document}

\advancethree{one}{two}{three}
\begin{equation}
1+1=2\qquad
2+2=4\qquad
3+3=6
\end{equation}\restore
Another equation
\begin{equation}
4+4=8
\end{equation}
Here are the references: \ref{one}, \ref{two} and \ref{three}.

\end{document}