\makeatletter
\renewenvironment{theindex}
{\pagestyle{plain}\parindent\z@
\parskip\z@ \@plus .3\p@\relax
\let\item\@idxitem}
{\thispagestyle{plain}}
\makeatother
\begin{multicols}{3}[\chapter*{Index}\addcontentsline{toc}{chapter}{Index}]
\printindex
\end{multicols}
