First of all, I think it is wonderful that you have decided to neuter your dog, and five or six months is a great age to do it. How do we keep a powerful breed humble, surrendering, submissive, and peaceful? We never reward excitement. We don't favor any one dog over another. Once you have a pack, you can't favor any particular dog. You need to favor the calm-submissive state of mind. That's what keeps a pack healthy. When you're feeding three dogs, always feed first the one that is the most calm and submissive. If that happens to be the pit bull, then feed him first. But if he's too exited, he should eat last. Wait until he's achieved the state of mind that you want. That way, he understand that it's the calm-submissive state that makes him part of the group.
If you have a high energy pit bull and lower energy Labradors, then make sure that you challenge the pit bull more than the other dogs. Always keep powerful breeds very, very tired. When you're taking them on a pack walk, the most calm and submissive dog gets the leash first. When you open the door, who goes out first? Obviously, the human and then the most calm and submissive dog. If all three of them are in a calm-submissive state, which is ideal, then you can take everyone at the same time! If you have problems walking all three dogs together, then take them one at a time. Master the walk that way. Then add a second dog to the walk. Then once you have that down, bring the third along. Your goal is to make them a calm-submissive unit, working together. Stay calm and assertive.