I agree with this. French is definitely best at fullback (compared to at other positions), with his speed, skill and instincts. Because there is so much expected from a fullback there is less need of go forward there. Likewise because there is less required from wingers you can expect them to be good at taking hit ups. This is French's weakness.
Hayne is obviously a fast, powerful runner, with the size to make great starts on exit sets. This is why I think his best position is wing. But then he has good brains, skills and a kicking game, which are wasted on the wing (and centre). But he lacks the work rate for fullback.
Gutherson is the least dynamic of the three. He is also a smart player (ironically because he seems like a bit of a dope off the field), very strong and an excellent defender. But his strength is his work rate. This means he isn't perfectly suited anywhere. He is best at fullback or centre, but also wasted at both positions (his defence is wasted at fullback and his work rate is wasted in the outside backs). He's a bit slow to shine on the wing.
What I think will happen is that we'll do something similar to when Hopoate and Morgan played on our right side, with Morgan defending at centre and attacking on the wing. I'd say Hayne will attack at centre and defend on the wing, with Auva'a defending at centre (and attacking outside Hayne, obviously). I think we'll see Gutherson at fullback (when fit) and French on the wing again. This will make us great against the high ball, but teams will kick high to French hoping to trap us in goal or at least in the corner.
1. Gutherson
2. French (right)
3. Jennings (right)
4. Hayne (left)
5. Auva'a (left)