This sounds like typical pack behavior. If I read your question correctly (I am not sure I do)...
Your 3 year old dog has behaved well and was with your senior dog since a pup. The gender of your dogs were not mentioned; however will make a difference.
If your three year old feels his position in the pack is being threatened by the new puppy. He may see his only recourse is to take over the Alpha position (perhaps by threatening the senior dog) If your 3 year old was a puppy when you brought him into your home; your senior dog would automatically be imprinted as the Alpha. Occasionally, this imprinting will happen if an adult dog has been around other dogs that have hurt him.
There are a lot of variables that need to be considered. There are people trained in dog behavior that can help you with this. They will need the full history on all three of your pets. Ask your Vet or local Humane shelter for the name of a trained dog behavioralist in your area.
This will be okay, dogs can be re-trained. But they do have natural pack behavior that is hard to work with if they are all males ( There is a term used, "top-dog" that originates from this behavior).
They may surprise you and work this out on their own. Just keep an eye on the fighting especially with a senior dog involved.
God Bless and give you patience,
QT