How to train my dog not to bully other dogs?

My dog is pretty obedient, though she has just entered the stubborn teenage years (6 months old).

I was doing some training at my parents house with my dog and two other dogs. My goal was to practice recalling my dog and doing some other basic tricks (sit, jump, speak) while she is excitedly playing with other dogs. And despite the chaos of three medium to large size dogs all wanting my attention at the same time, I felt that it was going quite well. Then I noticed my dog bullying the other dogs.

As I was training, I felt obliged to also give treats to the other two dogs who where obediently sitting and jumping as I gave commands to my dog. Well, my dog is already bigger then the other two dogs and was using her size to push the other dogs away when I was giving them treats.

I don’t like this behavior. I’m fine with them sorting out their own hierarchy when playing together, but when I (or anyone else) is playing with/training the dogs I would like my dog to behave "politely". In other words, if I give a treat to another dog I want my dog to sit and watch nicely instead of body checking the other dog and nipping at the other dog to push her away.

The other two dogs are 3+ years and do not show this behavior.