\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{natbib}

\begin{document}

Some silly simple file~\cite{test}.

    \include{sub/file1.tex}
\input{sub/file2.tex}
\input{sub/file3.tex}\par

\bibliographystyle{plainnat}
\bibliography{references}

\end{document}
