I just did this one for the challenge. I just put the guide lines on pixel lines 800 & 1600 both vertical and horizontal (I scrap 8x8) and then made sure things were placed along those lines.
I think depending on your rpogram you can set up guidelines to split your page into thirds. I do 12x12 so mine would be at 1200 and 2400 I believe? Then just place the focal point of your layout somewhere on the interesection of one of those points and place your embellishments so they are set up in threes and also at intersections of the layout and that should meet the requiremebts of the challenge