It certainly does sound like there is some stress in your household amongst the dogs and the new pup seems to be the trigger. It seems like Tag as a young entire male, is trying to be the dominant top dog and trying to show the pup who is boss. There are some measures we can take to try and help the transition and I think all 3 dogs would benefit.
There is a pheromone product called Adaptil, which is available as a plug in diffuser and can help calm dogs to enable them to adapt to the new situation. It would be best used in the area where Tag spends most of his time.
There is also a supplement called Zylkene which can be effective and is best used in combination with Adaptil.
It is a milk protein extract and works by promoting a feeling of contentment - think a young pup with a belly full of milk after a good long feed.
These measures can help the dogs transition and can help keep the peace a little.
You will also need to work to help the dogs tolerate each other, however we may need to come to terms with the fact that they may never be the best of friends.
Initially, you could try feeding Tag and the new pup on each side of a closed door, so they can smell each other but not feel threatened by each other. This can really help to get them used to each other in a non confrontational way.
Gradually, if things start to improve, you can try introducing them in the same room for short, supervised periods. This will best be done when both dogs are tired - after exercise is an ideal time to try this. Exposure should be very gradual and controlled, if the pup is jumping all over Tag, it is not going to go well.