Test of three-author mn2e support. See https://github.com/timj/mn2e-bst/issues/10

mn3-test.tex

% We must have both mn2e.cls and the repository mn2e-bst.sty in the TEXINPUTS path.
% Run latex on this file, and observe that the 'first-use' form appears once in the text,
% and the 'short-form' thereafter; and observe that only the 'short-form' appears in the caption.
\documentclass[usenatbib]{mn2e}

\usepackage{mn2e-bst}

\begin{document}

Here is a document.

\begin{figure}
Boo!
\caption{We refer to \citet{threea} and \citet{threeb} in the caption}
\end{figure}

We refer to both
\citet{threea} and \citet{threeb}.

Then, for the sake of variety, we refer to
\citet{threea} and \citet{threeb} again.

\begin{thebibliography}{}

\bibitem[\protect\citeauthoryear{One, Two \&
  Three}{\protect\mniiiauthor{threea}{One, Two \& Three}{One
  et~al.}}{2001a}]{threea}
One A.,  Two B.,    Three C.,  2001a, MNRAS, 3, 301

\bibitem[\protect\citeauthoryear{One, Two \&
  Monkey}{\protect\mniiiauthor{threeb}{One, Two \& Monkey}{One
  et~al.}}{2001b}]{threeb}
One A.,  Two B.,    Monkey C.,  2001b, MNRAS, 3, 302

\end{thebibliography}
\end{document}