Printing a trifold spread

I'm assuming this is a letter-sized (8.5x11) layout, to be printed both sides and tri-folded.

Your document page should be Letter, 8.5x11, and your layout should fill it edge to edge. If you have margins to accommodate a home or office printer, they should be inside the page size and about a quarter inch wide (most printers will get that close to a page edge; you might have to increase them slightly to prevent any clipping).

If this is for some form of commercial printing and the layout 'bleeds's off the edge, you should have a bleed defined all the way around of about 1/8 inch, and any color or graphics intended to go to page edge should extend to this outer margin.

When you print (to a local printer), you should select Letter sized paper and no scaling or "to fit" or "fit all" — it should be 100%, or print all, or unscaled. That should print it all exactly as proportioned in your layout.

When you export to PDF for commercial print, be sure to have bleeds enabled in the export menu ("Use Document Bleeds") and your PDF will come out with your layout at full size, but with an overall size including the bleeds, or about 11.25x8.75.

Does that give you enough to go on? If not, describe your setup and print method in a little more detail.