Q:

Change counter equation

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}

A:

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}

