Multiple equation number for a single equation with multiple labels to be referenced by

Multiple equation numbers can be assigned to single equation using the following code and also the equation numbering is continues and how can this equation with multiple equation number can be assigned multiple labels that can be used to reference this equation?

The equation numbers can be coded using this code, when it comes to labeling the equation it does not take even a single label.

    documentclass{article}
usepackage{mathtools}
usepackage{amsmath}
begin{document}

begin{equation} label{eq:1}
a+u=34
end{equation}

begin{equation} 
a+u=34
tag{refstepcounter{equation}theequation,refstepcounter{equation}theequation,refstepcounter{equation}theequation}
end{equation}

begin{equation} label{eq:5}
a+u=34
end{equation}

The equation ref{eq:1} and equation (ref{eq:2}) to ref{eq:4} are not refered as it number is displayed in the pdf

end{document}