One of those three has to go (Smith, Drummond and Monroe). Monroe is the obvious choice, but his agent has said multiple times that they're not resigning for anything less than $15 mil (which is a bit much IMO). I think he and his agent's stance makes it a little more difficult to move Monroe for what they'd want to get in return. They need a SF badly, so I could see them coming off of Monroe for Jeff Green and a future 1st.
However, if they can't get anything close to what they want from Monroe, they could always keep him and auction off Drummond the whoever offers the best package. I'm sure they'd rather part with Monroe over Drummond, but it may be in the franchise's best interests to go the other way. In all likelihood, you're getting a much better return for dealing Drummond over Monroe.
And of course, Smith... not a moveable contract there.