Are you absolutely 100% positive that your 3 year old has not been cycling "quietly" and been bred "quietly" by your stallion? Some mares do not show sings like squatting, peeing, winking, etc., but still cycle and ovulate normally, and some stallions, when given the choice, do not breed mares (or certain mares) with an audience present.
The reason I ask is that in most cases I have heard of or seen, when a mare is acting "studdy", it is because of hormone swings due to being pregnant. Another cause of "studdy" behavior in a mare is a type of cyst or tumor on an ovary.
Either way, you probably want to have this mare checked by a vet-- if she IS pregnant you can calculate a due date and begin prenatal care--- if she is NOT pregnant you can ask the vet to check for any reproductive concerns or abnormalities that would keep her from cycling/getting pregnant, and you can get those addressed, if your intention is to raise a foal from her.
If the 3 year old will not allow the stallion to pay attention to the other mare, you will probably have to split up the group as well and either just pasture the stallion with one mare at a time, or keep everyone separate from the stallion and hand-cover each mare as they cycle.