If he was making these same shots in practice last year playing in college then yeah i wouldn't be too confident he can make them in real game situation. But i doubt that was the case and he was missing in both practice and game situations.
Well, he did shoot 35% from 3 last year in games. And he was 4-6 in March Madness.
All he needs to be able to do is hit the open 3, and he's shown plenty of signs he'll be able to do that, especially with the countless hours we know he'll put in.
And he doesn't even need to be able to hit the open 3, in the highly unlikely event he never puts that shot together.