While I wish MS would provide sales numbers for the SP3 (all variants), it is unlikely to happen. This makes it difficult to judge how well they have sold. But the general buzz is very good about the device. So, I am not surprised that the SP3 is sold out in some places with the caveat - as others have mentioned above - that this could merely be a function of the number of units ordered by the retail outlets.
That said, I am currently traveling in Asia - South Asia to be specific and here there is no sign of the SP3. Earlier this evening I met a few people who saw my SP3 and were positively drooling. But I know and they know that MS has not released in their market. I think this is a mistake. Yes, the SP3 would probably be ridiculously expensive here, but then again the people who would buy it are not the types who would feel the cost - moreover, their companies would probably buy it for them. Small though the numbers may be, I think selling the SP3 with very good support can make the SP3 an aspirational device in such markets, which is one way to out-do Apple, whose products are also outrageously expensive here.