Too early to say really on McCain. Cam Thomas to me is a losing player--a guy who can score inefficiently and doesn't do anything else and also needs the ball a lot to be effective. If he eventually gets on a good team he could be a Jamal Crawford/Lou Williams type, but I just don't think his style or his attitude are conducive to winning. Crawford and Williams had a professionalism and comfortability coming off the bench as scorers for very good teams, something that it underrated and why a lot of talented-but-flawed scorers never materialize into having the kinds of careers Crawford and Williams had.
Pritchard we know can play a valuable role on a title winning team and can be effective off-ball. None of these guys are going to be good defenders, but I'd wager Pritchard is the most active and intelligent in terms of knowing where to be, albeit the most physically limited. Pritchard is the oldest, but also has shown a great trajectory of consistent improvement dating back to his college career. He has gotten notably better pretty much every year.
McCain is off to a torrid start, but most likely is going to come back down to Earth. Right now he would probably have the highest ceiling of the three players, just based on how he has started and it doesn't seem crazy to think he could one day be a legit star player in the league if he can find this level consistently.