I go by the rule that the CLO number is to Synthetic insulation as inches of loft is to Down.
Your 3 layers of 5 ounce XP, by my thinking, gives you a total CLO of 12 or so. In my way of thinking that is more or less equal to 12" of loft for Down. Think about sleeping under a Down quilt with 12" of loft on top of you.
My "0" degree XP and Combat sleeping bags will each use 1 layer of 5/4.7 ounce insulation. At "0" or so I would be wearing my Cuben VB pants, shirt, and booties. Over that I would have on my Patagonia #2 wool pants, short neck zip hoodie, a pair of silk-weight glove liners, one pair of my hiking socks and if necessary a set of silk insulated pants, shirt/jacket, and maybe my Combat footies. This is about what I would be carrying and I still have my silk insulated vest and rain gear to put on if necessary. I will be sleeping on my Warmlite Down Air Mattress which goes into my Bivy.
When it is 20 degrees or less the Cuben VB stuff stays on all the time and can be vented as necessary during the day while hiking. The silk insulated items can be worn, vented, or removed if I get to warm. One of these days I am going to make a set of pants and shirt / jacket out of Pertex Shield to wear as my outer garment to cover the Cuben VB stuff. This is a new WPB material from Pertex. I only have a 3 yard sample of it so I need to be sure to use it to it best advantage. I can get more but only in amounts of 550 yards. The Pertex Shield items will also be my rain wear. The Pertex Shield material weights about 1.9 ounces per sq yard so it will be no zippers (snaps) and close fit.