Sometimes, teammates push around new recruits because they perceive them as easy marks, so to speak. This may be the case with your new buneary, as not only are members of this species bred to be rather cute and docile, but also, given the typical dispositions of espurr and sylveon, I wouldn’t be surprised if either of yours were attempting to assert some sort of dominance. Think of it as your espurr and sylveon showing the newcomer that you prefer them, in a way.
While team building and mediation techniques are still most certainly important (to make it clear to all members of your party that they are a team and that you expect them to treat one another with respect), what may be more effective is simply training your buneary a bit. Teach it moves to defend itself and make it be a valuable part of your team’s day-to-day activities. If you battle, then most definitely allow your buneary to battle as well. The more you train your buneary, the more its confidence will grow, which in turn will allow it to assert itself to your espurr and sylveon.
Best of luck, anonymous!
The Pokédex According to Bill 