testfancycenter.tex

\documentclass[openany]{book}
\usepackage{lipsum}
\usepackage{calc}
\usepackage{boxedminipage}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\fancyhead[c]{\fancycenter{AAA}{BBBBBBBBBBBBBBB}{CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC}}
\fancyheadoffset[LE,RO]{\marginparsep+\marginparwidth}

\fancypagestyle{plain}{%
    \fancyhead{} % get rid of headers
    \renewcommand{\headrulewidth}{0pt} % and the line
}
\begin{document}

\tableofcontents

\bigskip

\begin{boxedminipage}{\textwidth}
This document is a test for the \verb|\fancycenter| command. It puts a header with 3 fields in an extended header, including the margin par area. It also redefines pagestyle \texttt{plain}.
\begin{verbatim}
\fancyhf{}
\fancyhead[c]{\fancycenter{AAA}{BBBBBBBBBBBBBBB}
               {CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC}}
\fancyheadoffset[LE,RO]{\marginparsep+\marginparwidth}
\end{verbatim}
\end{boxedminipage}

\chapter{Introduction}

Here comes a marginpar.
\marginpar{Donec bibendum quam in tellus. Nullam cur- sus pulvinar
  lectus. Donec et mi. Nam vulputate metus eu enim. Vestibulum
  pellentesque felis eu massa.}
\lipsum



\section{The Problem}
\label{sec:problem}

Here comes a marginpar.
\marginpar{Donec bibendum quam in tellus. Nullam cur- sus pulvinar
  lectus. Donec et mi. Nam vulputate metus eu enim. Vestibulum
  pellentesque felis eu massa.}
\lipsum[1]

\section{Evaluation}

Here comes a marginpar.
\marginpar{Donec bibendum quam in tellus. Nullam cur- sus pulvinar
  lectus. Donec et mi. Nam vulputate metus eu enim. Vestibulum
  pellentesque felis eu massa.}
\lipsum

Some more text.

Some more text.

Some more text.

Some more text.


\section{Another section}

Here comes a marginpar.
\marginpar{Donec bibendum quam in tellus. Nullam cur- sus pulvinar
  lectus. Donec et mi. Nam vulputate metus eu enim. Vestibulum
  pellentesque felis eu massa.}
\lipsum[3]

\end{document}