Of the 3 you mentioned, I think Cowart has the most talent though being drafted out of HS, he may not see the majors for 3+ years. If you're okay waiting that long, I'd say he's the pick here. Harvey, if he stays healthy, would be second and will likely see the majors much sooner. I don't see him on your lists, but is Kolbrin Vitek available?
Here is a small write up I did on Vitek...
Vitek is playing 3B longterm and the Red Sox view him as a their future 3B there. There really isn't anything not to like about Vitek imo. He'll have a 6'3 225 peak body which will increase his power, quick hands, good barrel control, double-digit steals, glove will translate to 3B, has a killer arm as he pitched low 92-94 in college, and he was also picked over Cox in the draft by the Red Sox, who aren't dummies. http://web.minorleaguebaseball.com/news
Pay special attention to the link which shows that Vitek's minor league performance was much better than that of Brentz.