Doesn't make any sense for them to be on the floor at the same time. You've got 48 minutes to fill at the 5. Playing them together would effectively increase that figure. Ideally Shaq would go at least 20 minutes per game and at least 24 for JO. Any left over minutes would go to Erden.
Doc has also said Shaq will get some back to back nights off, so Erden will get some significant minutes in this case.
Erden is getting lots of minutes in preseason, but I don't see him being a big part of the rotation if both Shaq and JO are healthy and in the lineup.