It could possibly be related to heat, but it could also be a young dog trying to establish her spot in the pack when she is not used to a stable dynamic. With two shepherd-type dogs, there is an increased risk of pack mentality taking over. I would make sure your terrier is separated from the two big dogs if you are not right there beside them. It is also important that you clearly establish that you are alpha and you won’t tolerate aggression in the pack. Shepherds and huskies both feel insecure if there is not strong leadership. She would probably really benefit from a job like agility or running.
Your primary responsibility is to your existing dogs so if the terrier is in physical or emotional danger, you need to address that immediately.
I am not the right personality type to lead such strong-willed dogs, so I understand if it’s hard for you to step up to this challenge. If you’re also not a strong alpha, it may be best to find another home for the new pup. Good luck!