• Try \addtolength\oddsidemargin{3mm}\addtolength\evensidemargin{-3mm}. This also accounts for double sided printing. If you are doing single sided, then you want both even and odd to be +3mm. – Steven B. Segletes Sep 10 at 10:56
• In what sense, "not working"? Here's an MWE, where it kicks in on pages 3,4 \documentclass{book} \usepackage{lipsum} \begin{document} \lipsum[1-9] \clearpage \addtolength\oddsidemargin{30mm}\addtolength\evensidemargin{-30mm} \lipsum[1-9] \end{document} – Steven B. Segletes Sep 10 at 11:08
• Perhaps \geometry{bindingoffset=3mm} from the eponymous package? – Bernard Sep 10 at 11:13