I got the 3 machines setup and booting off a usb drive... but I dont like the slow writes of the drives. (the usb drive was used so I could use all the sata ports for storage hard drives)
I cant find a decent priced pcie ssd nor a budget mpcie ssd....so: I think I may change it around to have the nodes run root from an ssd with iscsi and just boot from the usb drives.