Thank you for that information. Because they are females she may be trying to establish herself as the top dog among the other two. most likely it is directed at one of them she perceives as being stronger and the other one is just a casualty.
I would start basic behavior training daily so she sees the humans on top and not the dogs. I would also introduce the learn to earn program to her where she must work for you for anything she wants.
I would then begin behavior modification training with her.
You may be trying to rush it inside the home as we know they do not get along, so I would keep taking them to areas where they do get along.
There may also be a bit of possession aggression creeping in and this gets brought to the forefront in an enclosed area. She may feel possessive of your girlfriend and seeing her around the other dogs in close proximity may be a set off point.
below are links for training in a positive manner so she can succeed , and to the learn to earn program I talked about.
Try to get a firm leave it command in place so that you may be able to stop her from going after one of the other dogs.
training must be done daily and try not to bring them together inside the home until she commands are firm inside the home. You can do the BAT inside by putting two dogs at a time together with leashes on and rewarding them for good behavior. I would put the third dog in a different room while you do this. this will also help you to see which dog she really has a problem with.
Clicker training/positive method training/ print off
video’s to see how clicker training is done,(go to videos )
Dr sophia yin learn to earn https://drsophiayin.com/blog/entry/the-learn-to-earn-program/
Training -Behavior adjustment http://empoweredanimals.com/