Well, she will not be nice to the new pullets, but because you are adding three to one, that will help. But there will be a dust up, however should not be terrible.
What does your run look like? Is it just an open rectangle, where as any chicken in the run can see all of the other chickens in the run? If so add some clutter: ladders, chairs, boxes, totes on the side, small pieces of plywood, roosts, pallets leaned against a wall or up on cement blocks. This allows chickens to get out of sight of each other, and allows you to have multiple feed places where a bird eating at one, cannot see a bird eating at another.
Then let the old bird out of the coop/run for the day, put the pullets in the coop/run for a day, if you can, feed along the fence. This allows the new birds to get familiar with the new place without being chased for their lives. Then as close to dark as possible, let the old bird in. The urge to roost will be nearly as powerful as the urge to fight.
Depending on the size of your coop, and its set up, sometimes it is helpful to add another roost, lower and out of reach of the top roost, where your old girl will reign until they new birds start to lay. They will not be one flock until the pullets start to lay, but should be able to be together. The young ones will be a sub flock.
Good luck,
Mrs K