All these guys have lots of potential, but they'll probably encounter some serious growing pains along the way. Of the three, Cox has had the best start, but that doesn't mean much this early.
If you're looking for trade bait, I'd say Ensberg has the best chance to develop into a productive player this year, but watch his playing time closely. He may platoon early on, which will sink his value for a while.
If you're looking to make a trade soon, go with Cox. Later on in the season, both Ensberg and Johnson should pass him in terms of value. One more thing to keep in mind is that a 3b should be easier to deal than a 1b.
Yikes, this is a tough call. Some more opinions on this would be nice, but for what it's worth, I'd take Cox, but keep a close eye on Ensberg. If it looks like he's getting hot and will play every day, go get him.
(Unless this is a keeper league, in which case Johnson is your man).