megvan a megoldás! (:
a tied alapján. működik!
\documentclass{book}
\renewcommand{\thesection}{\arabic{section}}
\renewcommand{\thesubsection}{\arabic{section}.\arabic{subsection}}
\renewcommand{\thesubsubsection}{\arabic{section}.\arabic{subsection}.\arabic{subsubsection}}
\setcounter{secnumdepth}{3}
\begin{document}
\chapter{Test chapter}
\section{Test section}
\subsection{Test subsection}
\subsubsection{Test subsubsection}
\end{document}