Change counter equation

Question

I am relatively new working with LaTeX and I don't know how to solve this problem.

I have several equations that I wrote in a general form, but when writing them, I want to show that the general form is just a generalization for three specific formulas; i.e.

I have this

but I want to have this:

P.S: I could use the command tag like in this post Customize equation numbering for Equation environment?, but I have over 60 equations and I could not use the command tag for each line individually (Also, there could be multiple errors doing it manually) Can someone help me please?

My MWE is:

  \documentclass[a4paper,12pt]{article}
  \usepackage[spanish]{babel}
  \usepackage[latin1]{inputenc}
  \usepackage{amsmath}

  \begin{document}

  \begin{equation}
   E=mc^2 
  \end{equation}

  \end{document}

EDIT: For more clarity, I put some equations with subscript (j) and others not; and those with subscript I want to be counted not as just one equation; but from n (number of previous equation) to j (from 1 to 3).I know how to set this MANUALLY, but that wouldn't be efficient nor exent of mistakes since we are talking of over 100 equations in different sections). An example is:

My MWE is the following:

\begin{document}

\begin{equation}
 F=ma
 \end{equation}

 \begin{equation}
 E=m_{j}c^2 \tag{2), (3), (4}
 \end{equation}

 \setcounter{equation}{4}
 \begin{equation}
 A=B+C
 \end{equation}

 \begin{equation}
 D_{j}= E_{j} + F_{j} \tag{6), (7), (8}
 \end{equation}

 \end{document}

Answer 1

I'm not sure it's a good idea, particularly if you never refer to a single case. You can just refer to “Equations (1)”, which will give readers the hint that they're more than one.

Anyway, here's an implementation that also allows to refer to a single equation from the bunch.

\documentclass{article}
\usepackage{amsmath,xparse}

\ExplSyntaxOn

\NewDocumentCommand{\multinumber}{mo}
 {
  \addtocounter{equation}{-1}
  \seq_clear:N \l__ignacio_multinumber_seq
  \int_step_inline:nn { #1 }
   {
    \refstepcounter{equation}
    \IfValueT{#2}{ \use:c{ ltx@label } {#2-##1} }
    \seq_put_right:Nx \l__ignacio_multinumber_seq { (\theequation) }
   }
  \tag* { \seq_use:Nn \l__ignacio_multinumber_seq { ,~ } }
  \addtocounter{equation}{1}
 }

\ExplSyntaxOff

\begin{document}

References: \ref{Einstein} (global), \ref{Einstein-1} (first)

\begin{equation}
F=ma
\end{equation}

\begin{equation}\label{Einstein}
E=m_{j}c^2 \multinumber{3}[Einstein]
\end{equation}

\begin{equation}
A=B+C
\end{equation}

\begin{equation}
D_{j}= E_{j} + F_{j} \multinumber{3}
\end{equation}

\end{document}

Answer 2

This allows you to add multiple equation numbers via a command \multitag.

\documentclass[a4paper,12pt]{article}
\usepackage[spanish]{babel}
\usepackage[latin1]{inputenc}
\usepackage{mathtools}
\newcounter{iloop}
\newcounter{imax}
\newcommand{\MultiNum}{\edef\temp{}%
\setcounter{iloop}{0}\loop%
\stepcounter{iloop}%
\ifnum\value{iloop}<\value{imax}%
\edef\temp{\temp\number\value{equation}),(}\refstepcounter{equation}%
\repeat%
\edef\temp{\temp\number\value{equation}}%
\refstepcounter{equation}}
\newcommand\multitag[1][1]{\setcounter{imax}{#1}%
\MultiNum%
\tag{\temp}}

\begin{document}
Einstein 
\begin{equation}\label{eq:Einstein}
 E=mc^2 \multitag[3]
\end{equation}
Photon
\begin{equation}\label{eq:Photon}
 E=h\nu \multitag[2]
\end{equation}
Thermo
\begin{equation}
 E=TS
\end{equation}
Wrong
\begin{equation}\label{eq:Wrong}
 E=mc^3 
\end{equation}
\eqref{eq:Wrong} is wrong, \eqref{eq:Einstein} is correct, and so is \eqref{eq:Photon}.
\end{document}

Answer 3

Not sure if it is a good idea use a tag as a reference, because then, it should be counted or not? Is 1+2+3=4 or it is simply 1+2+3? What the reader must expect for the next equation, 4 or 5? That is, the sequence must be 1,2,3,1+2+3,4,... or maybe 1,2,3,1+2+3(=4),5...?.

Anyway, for the first case, is possible using \label{} and \ref{}, and for the second, you must add also a \addtocounter{equation}{1} in the 4th (1+2+3) equation:

\documentclass{article}
\usepackage[spanish]{babel}
\usepackage{amsmath}
\usepackage[colorlinks]{hyperref}

\begin{document}
Siendo la masa  ...  

\begin{equation}\label{a}
 m = \gamma m_0 = \frac{m_0}{\sqrt{1-v^2/c^2}}
 \end{equation}

Y el momento linear es bla bla ... 
\begin{equation}\label{b}
E = \sqrt{p^2c^2+m^2c^4} = \gamma mc^2
\end{equation}

Por lo dicho anteriormente, ....
\begin{equation}\label{c}
\gamma = \frac{1}{\sqrt{1-(\frac{v}{c})^2}} 
\approx 1+ \frac{1}{2} \left(\frac{v}{c} \right)^2
\end{equation}

Entonces, juntándolo todo   \eqref{d} llegamos a:
\begin{equation}\label{d}
%\addtocounter{equation}{1}
 E=mc^2   \tag{\ref{a},\ref{b},\ref{c}}
\end{equation}

Lo que está tan claro como que 2 y 2 son: 

\begin{equation}\label{e}
 2+2=22
\end{equation}


\end{document}