aggression problem

That defintiely sounds like one for the pros. At first one might attribute it to protection but they don't come a whole lot more protective than a Rottie so perhaps it's an alpha thing. I don't claim to understand everything I know about the aplha deal but some dogs seem to want it and some don't. I once had a GSD and a Bloodhound. Both extremely dominant, they would fight every chance they got and the GSD would soundly kick the BH's ass every time but he kept coming back for another ass kicking everytime. Finally had to give the BH away.
I got my previous GSD from a friend who had gotten him from a friend who could no longer care for him. The guy bought another GSD puppy and within a year the pup was making life a living **** for the older dog. It wasn't enough to be simply dominant, Saul didn't give flip either way, he wouldn't let him eat, drink, be around anybody else and I'm sure he begrudged him the air he breathed.
Saul could have easily dispatched the pup but it just wasn't the way he was. Good luck with yours.

You are in exactly the situation that scares most of us away from getting littermates or pups the same age. They can all have the same problems and issues at the same time! (Click here and Click here )

I know some people end up playing 'The Crate Game' and never being able to have all dogs out at the same time. I personally would find that hard to do, but if it was the only way to prevent medical bills and the stress/drama of having all the dogs out, it would either be that or me finding a new great home for at least one of the dogs.

Hopefully you can find a trainer/behaviorist to help.

I know that I'd be trying to REALLY up the exercise for everyone (specially if one dog seemed more the instigator). As well as working on leadership (not just 'obedience' but real leadership that our dogs understand) for myself in the pack you have created. A good book is The Dog Listener by Jan Fennell. Also some good articles at http://www.flyingdogpress.com/articles.html

I think it may be time for more classes for all of them or at minimum the instigator (your GSD). Also, check out the link in my signature for information on being a good leader; leadership makes a huge difference. It sounds like the GSD is alpha in your pack and is trying to dictate when and who can interact with you. Big no-no! YOU are the leader, you have every right to deflate your GSD's ego and make it crystal clear that he does not decide how the pack interacts with you; only you have that right. A good solution is to greet your GSD first, send him away, greet the next dog (probably the pit) and push away the GSD and correct him strongly if he comes too close, then send off the pit and greet the rottie. In this way you respect their order but you also make it very clear who calls the shots when it comes to giving attention and affection. Finally, I agree with all the above posts. Separation is a big thing; some dogs just get too competitive. It would be wise to not allow them to be alone together ever, lest you want to come home to a veterinary emergency or a dead dog.

What do you do to teach them to get along?