Sanheim is the most talented of the three, but because his game requires risk taking, he's also the one they have to ride the hardest.
He developed bad habits in juniors (which he readily admits) and has to stay focused on when to take risks, and to skate as hard getting back as he does attacking.
They're just bringing him along slowly, and trying to avoid putting too much defensive pressure on him, for one thing he lacks strength to fight for the puck along the boards and the corners, and he's not a shot blocker.
This will all come to him over the next couple years, he'll get stronger, understand his defensive responsibilities (as he's said, in juniors he was just told to go make plays), and learn what he can get away with in the NHL.
To me, most talented offensive players who come out of junior or many college programs are similar to mobile QBs coming out of college, in that they're going from a situation where they were told to just be athletes to one where almost everyone is an athlete and you have to function in a more structured environment and understand the game and your responsibilities. While hockey is less structured and more fluid, to function well you have to read the defense, know what can work, understand how they're attacking you and who is your responsibility. And that takes time and discipline to master. So coming from a situation where a coach just told you to skate and make plays, to one where you are expected to play with fundamentals and positioning, is a big jump for a lot of players. Look at Babcock demoting and benching top picks in Toronto to try to get that message across. Or Drouin getting sent back to the AHL.
It's easier for players like Morin and Hagg, they have fewer responsibilities and are taught to play it safe (because if they get caught deep, it can quickly get ugly).
To be fair to Manning, two years ago he was totally lost, used to playing the Sanheim/Ghost role in the AHL and just learning that he wasn't good enough to pull that off at the NHL level. He's become much more reliable, doesn't get caught deep as much, fewer bad pinches, etc. He's turned himself into a passable 3rd pair defenseman.