Keep them all at least until Kemp shows that he's healthy and productive - or the Dodgers acquire someone who can be a starting CFer.
While I'd prefer to keep Ethier and move Crawford it might make more sense to move Ethier first and let Crawford build more value by playing everyday.
As for Joc. There's no rush on him.
But Steamer, Oliver and ZiPS combine to project him for a .247/.317/.406 line and 2.9 WAR per 600 PA's - which is better than either Ethier or Crawford. He's very close and I'd expect him to, at minimum, get a Sept callup this season.