The people selling AT&T Pre3's on ebay are HP employees, or know an HP employee, and got them from the HP webOS store. There is no other way to get them. HP is apparently providing unlock codes with each phone, as every single AT&T Pre3 I've seen listed comes with an unlock code (or is already unlocked). The AT&T Pre3's have no warranty or support from HP or AT&T (officially they don't exist), but work just fine.
Originally Posted by SnotBoogie
There are also UK & Euro spec Pre3's for sale, which came from various retail channels in europe. These do have support and warranty from HP.
There are a small number of Verizon Pre3's trickling out as well, at very high prices. These come from the same source as the AT&T Pre3's - HP employees buying them from HP's webOS store, and again have no warranty or support. There were rumors that Verizon would not activate these, but several people so far have purchased them and activated them without issue. Another new rumor says that HP has stopped (or will stop) selling Verizon Pre3's, but this could just be a means to keep the ebay prices up... still, surprisingly few have come up for sale.
Sprint won't activate Verizon phones, and vice-versa. So there will be no Pre3's running on Sprint.