There is 0% chance Boston would do this... why turn a guy who they put in so much time into, is an elite defender, and has improved his shooting?
I take it you voted "yes".
The only part of this that's both true and relevant is that Smart is an elite defender. The Cs have other truly elite wing defenders, not to mention recent draft picks at Smart's position.
So, there are a variety of reasons they might. Two examples:
1) they find a nice deal for a veteran guard that includes the 3 pick but doesn't include Smart, but see a draft prospect they believe has high upside at 5.
2) they can't find a good deal for 3, take another guard with that pick because they deem him BPA, and still see a draft prospect they believe has higher upside than Smart at 5.
I don't buy the Cs wouldn't trade Smart because he's played there for 2 years. And I'd be absolutely shocked is Ainge and Thibs don't talk deal in some way shape or form before the draft.