\documentclass{article}
\usepackage{fancyhdr}
\pagestyle{fancy}
\lhead{}\chead{- \thepage\ -}{\rhead{}
\lfoot{}\cfoot{}{\rfoot{}
\textheight=10cm% only for demo
\newdimen\zahl
\makeatletter
\renewcommand{\thepage}{%
	\zahl=\value{page}pt
	\ifdim\zahl<100pt
		\ifdim\zahl<10pt 
			00\arabic{page}%
		\else 0\arabic{page}%
		\fi
	\else\arabic{page}%
	\fi%
}
\makeatother

\begin{document}
blah
\newpage
blah
\newpage
blah
\setcounter{page}{10}
blah
\newpage
blah
\newpage
blah
\setcounter{page}{100}
blah
\newpage
blah
\newpage
blah
\end{document} 