For now, the XFCE ISO's only contain the XFCE desktop environment. Some day if I get XFCE all polished up and I get bored of tinkering with it, maybe I'll try implementing something very light, like fluxbox+fbpanel or openbox+tint2, or something very heavy, like Gnome3. At present, I'm pretty far away from getting bored with XFCE, though

I also think brokenman and fanthom might kick me out if I add a fifth DE to the mix
For what it's worth, one could easily drop the XFCE modules (0090 and 0091) from the XFCE ISO into the /porteus/modules folder of a standard Porteus install, and have all three DE's available from one installation. I have that on my flash drive presently, for testing purposes.