That would only allow for 999 possible box numbers. The number is unusual but legit. It appears on at least one of the REAL documents from Bush's records.
Actually, three-digit PO Box numbers would allow for 1000 PO boxes in each zip code, which seems like it would be plenty to me, but if the number appears in a genuine document, then I guess what I read was wrong, and I'll stop looking for the source.