This is a very confusing phone.......
The only place I've seen the 3-carrier version is from Amazon. (Out of stock, now). There are unlocked GSM variants. One seller has an X Charge listed as 4 carrier. Not enough model # info for me to tell if any of them are really the same phone.......
I did see a guess somewhere (slickdeals?) in a forum that the 3-carrier version may have been a run done for Sprint, who then decided not to follow up on offering it.
I haven't heard about any 4 carrier versions. I hope you can track down a 3 carrier for a good price, though. It's a great phone!