First, I think that the redundancy is Okafor and Embiid. They have similar back to the basket styles. Noel is a different body type. I think that, assuming Embiid is able to play, they will then move Okafor, since Embiid has more upside. I also think they will try their best to trade him to the west. The last place they want a former player to end up is with the Celtics, and Atlantic rival. When have the Sixers and Celts made a big trade together??
I'm not sure about that Out of those three guys, Embiid is really the only one who has a working jump shot right now. Both Noel and Okafor have zero offense outside the paint.
So that makes me thing that it's Okafor and Noel who are the redundant pair. Both guys really are pure centers, while Embiid (if he's ever healthy) probably has the mobility and the outside range to move over and play PF (Tim Duncan style) if he really had to.
Neither Noel nor Okafor can play the PF spot however - Noel is too lacking in range, and Okafor lacks the foot speed to defend NBA Power Forwards.
Out of the two guys to pair with Embiid, I'd say Okafor is the better fit. Noel is an elite defensive player, but Embiid also should be just as good defensively as Noel is. Offense however is the big question mark with Embiid - will he ever be more than a 12 - 14 PPG scorer in the NBA? Not much question about that with Okafor - he should be an excellent scorer from day one. Okafor is also a skilled passer, which neither Embiid nor Noel really are.
If I'm Philly I would take a chance on the Embiid / Okafor front court (which has the potential to be a better version of Memphis' Gasol/Randolph front court) and then try to trade Noel out in return for a talented young guard.
BUT I wouldn't do anything until I have seen Embiid and Okafor play at least 10-15 games and prove that they can compete at an NBA level. Until then it's just too much of a gamble trading out the only guy who has proven anything, and putting your team's future into two guys who have yet to play a game.