I don't think they've had a 3 day regular pass for some 8-10 years now. They might still have them for conventions though. You can always buy a 4 day and save the extra days. I guess you could buy off ebay but that seems risky to me. You could also buy the UMP option for the length of time you need.
check the budget or park board, there has been posts about a 3 day hopper, i think they bought them thru alamo , im sure this post will be moved there to.
good luck
Generally Ticket Mania has them for purchase. You can also check on the Mousesavers website and generally they will have a link to ticket mania. www.mousesavers.com
Call Central Florida Tickets and Travel and ask if they have any available (sometimes they do/sometimes they don't). 407 932-2080. Their shipping is kind of high but they deliver for free to local Orlando hotels. Usually they also give you coupons and maps too (ask if they still do this) .