So, if Embiid is on the board at 3, and the Celtics are offering their 2 picks this year to move up, who says no? It seems like Philly doesn't really have much use for Embiid, Exum, or Smart. Dropping down would probably position them perfectly to add Randle and a couple wings later in the draft and they'd have a pretty interesting, if extraordinarily young, roster.
Just to be clear, this hypothetical question only exists in a world where Wiggins and Parker are the top 2 picks. If either of those guys are left, the Sixers are not going to trade out as either player is exactly the sort of guy they need.