In Word, set the paper to Landscape and make a table of three columns to fill the whole of the page with the margins outside the table to suit. The centre column of the table is then set to a width to match twice the margin setting (so that it appears equal when folded. Set the table outline to invisible.
For a 4 page (single sheet booklet) the page numbering will be on sheet one, page 4 is the left hand column page 1 is the right hand column. On the second sheet (reverse of the first) page 2 on the left, 3 on the right. For larger numbers of pages, make up a dummy booklet, mark the pages with numbers and then dismantle to see how they are laid out.