File deel1.tex in directory deel1
\documentclass[a4paper]{article}
\usepackage{lastpage,xr,refcount,etoolbox}
\externaldocument{../deel2/deel2}
\externaldocument{../deel3/deel3}
\makeatletter
\patchcmd{\lastpage@putl@bel}{LastPage}{LastPage1}{}{}
\makeatother
\begin{document}
\tableofcontents
\section{First deel1}\label{first1}
\section{Second deel1}\label{second1}
\ref{first1} at page \pageref{first1}
\ref{second1} at page \pageref{second1}
\ref{first2} at page \pageref{first2}
\ref{second2} at page \pageref{second2}
\ref{first3} at page \pageref{first3}
\ref{second3} at page \pageref{second3}
\end{document}
File deel2.tex in directory deel2
\documentclass[a4paper]{article}
\usepackage{lastpage,xr,refcount,etoolbox}
\externaldocument{../deel1/deel1}
\externaldocument{../deel3/deel3}
\makeatletter
\patchcmd{\lastpage@putl@bel}{LastPage}{LastPage2}{}{}
\makeatother
\begin{document}
\setcounter{page}{\getpagerefnumber{LastPage1}}
\stepcounter{page}
\tableofcontents
\section{First deel2}\label{first2}
\section{Second deel2}\label{second2}
\ref{first1} at page \pageref{first1}
\ref{second1} at page \pageref{second1}
\ref{first2} at page \pageref{first2}
\ref{second2} at page \pageref{second2}
\ref{first3} at page \pageref{first3}
\ref{second3} at page \pageref{second3}
\end{document}
File deel3.tex in directory deel3
\documentclass[a4paper]{article}
\usepackage{lastpage,xr,refcount,etoolbox}
\externaldocument{../deel1/deel1}
\externaldocument{../deel2/deel2}
\makeatletter
\patchcmd{\lastpage@putl@bel}{LastPage}{LastPage3}{}{}
\makeatother
\begin{document}
\setcounter{page}{\getpagerefnumber{LastPage2}}
\stepcounter{page}
\tableofcontents
\section{First deel3}\label{first3}
\section{Second deel3}\label{second3}
\ref{first1} at page \pageref{first1}
\ref{second1} at page \pageref{second1}
\ref{first2} at page \pageref{first2}
\ref{second2} at page \pageref{second2}
\ref{first3} at page \pageref{first3}
\ref{second3} at page \pageref{second3}
\end{document}
Notes
We have to patch \lastpage@putl@bel in order to have different labels in different documents (the "prefix" strategy provided by xr would be more cumbersome). The command to patch if hyperref is used is \lastpage@putl@belhyper (same code).
The same scheme will work for citations if the xcite package is used.
xror the more modernzref-xrpackage (zrefbundle). In this case How can I reference a table/figure/section in another document? and referencing labels from other files would be related, but not that useful to you. – Martin Scharrer Sep 23 '11 at 08:01xrpackage, but how would I go about pointing it to the correct directory? Can I just use:\externaldocument{../DEEL1/DEEL1}or would this be too simple? – romeovs Sep 23 '11 at 08:13..) might not be allowed by the security settings. – Martin Scharrer Sep 23 '11 at 08:19xrpackage works! Great. Perhaps you should make it into an answer (I won't accept egregs answer, because part of it didn't work for me) – romeovs Sep 24 '11 at 09:52