Here's a solution using the pdfpages package. It consists of two steps: First you produce a document with a small paper size, and then you produce another document that combines these small pages onto one A4 landscape page.
This is your actual document, where you put all of your text etc. You might have to adapt the margins, page numbers, etc. to your needs.
% This file is called sixpages-doc.tex
\documentclass{article}
\usepackage[paperwidth=9.9cm,paperheight=10.5cm]{geometry}
% paperwidth is A4/3, paperheight is A4/2
\usepackage{lipsum}% just for filler text
\begin{document}
\lipsum[1-6]
\end{document}
This is the document that puts the six pages on one page:
\documentclass{article}
\usepackage[landscape,a4paper]{geometry}
\usepackage{pdfpages}
\begin{document}
\includepdf[pages=-,nup=3x2]{sixpages-doc.pdf}
% pages=- means all pages
% nup "Puts multiple logical pages onto each sheet of paper. The syntax of
% this option is: nup=⟨xnup⟩x⟨ynup⟩." (from the manual)
% If you want this layout:
% 1 3 5
% 2 4 6
% use the option "column".
\end{document}
This is the output of the second document:
