Shawn:
With the assumption that you're using CS5 or CS5.5, I'd take a different tact altogether. While also assuming the final piece is 11 x 8.5, create a document with two sets of three facing pages each 3.6875″ (3-11/16″) wide x 8.5 deep. You'll have to uncheck “Allow Document Pages to Shuffle” in the pages panel to arrange three pages side-by-side. Then with the page tool, change the shorter ends to 3.625″ (3-5/8″). That will make the total width 11″ with the shorter flaps 1/16″ less (1/8″ seems like overkill IMHO).
Now you can set up single columns/margins in each panel (though technically a page). When you export or print, just be sure to select spreads. And with this setup you won't have to draw in fold marks, just be sure to include the crop marks when exporting or printing.
BTW, if you select each three-page “set” in the pages panel, then uncheck “Allow Selected Spread to Shuffle,” you can then deselect facing pages in the document setup. You'll be presented with a dialog about maintaining spreads that you'll need to OK. Not really necessary, but it keeps things tidy if desired. Your pages panel should now look like this:

Bob