I think you guys are kinda over complicating the programs a little.
KC-X is just to replaces the first 175 KC-135 Tanker, with a much more capable medium size tanker. The Air Force will determine if a new platform type is warranted, or whether to continue with the current production aircraft of the KC-X for the KC-Y.
KC-Y will replace more of the aging tankers either based on the KC-X winner or a new yet to be determined aircraft.
KC-Z is supposed to replace the KC-10, but if the one plane they have can do the job of the KC-10, then they may just save the money and keep just one plane instead of 2 airframes.
But the way Congress has been cutting funding ie. the 183 F-22's down from 360+ that was wanted, or the 190 C-17's wanted to replace all the critical C-141's down from the 230+, to know wanting to replace all 481 KC-135's and 59 KC-10's. In the end the USAF may only end up with only 260 to 300+ tankers.
If Northrup gets it, then there is a 75% chance that they will get the KC-Y contract, if it is as efficient as it is supposed to be, then it would make since to have more A330MRT, that every 2 would do the job of 3 KC-135's or KC-767's. And at that point price would go down, and you now get to save money to put towards the KC-Z.