This is an interesting question, as it seems to infer that women are given positions of power or at least are able to express their power over others in these three stories. I would argue that the only women who are given positions of power are actually the minor characters. The protagonists of these stories actually show that they occupy positions that emphasise their powerlessness.
Consider Laura in "The Garden Party." As the younger daughter, she is bullied by both her sister and her mother in her response to the death of Mr. Scott. Note how her mother in particular responds to her pleas to cancel the garden party:
"You are being very absurd, Laura," she said coldly. "People like that don't expect sacrifices from us."
Laura is shown to occupy a position where in her limited sphere she has very little power at all, and where she is subject to the will of her older sister and mother.
In the same way, "The Little Governess" presents us with a naive and innocent figure, abroad for the first time, who is completely powerless. Her dependence on the German man indicates this as he steers her through her first day abroad. However, it is clear from the story that this governess is also economically dependent on Frau Arnholdt for getting a job, and the way in which she responds to being told that Frau Arnholdt had gone to the hotel and then left emphasises her lack of power:
"Where is the lady now?" asked the little governess, shuddering so violently that she had to hold her handkerchief up to her mouth.
Lastly, "Her First Ball" presents Leila as again a figure who is subject to the influence of others. The way in which the fat man who dances with her is able to upset her mood and depress her indicates her powerlessness. However, Leila, alone amongst the protagonists of this story, could be argued to exercise power in the way that she recovers from her painful epiphany and goes back to enjoy the dance, forgetting about the inconvenient truths the fat man shared with her:
But in one minute, in one turn, her feet glided, glided. The lights, the azaleas, the dresses, the pink faces, the velvet chairs, all became on ebeautiful flying wheel.
We could argue that Leila is thus the only protagonist who is shown to exercise power by her ability to forget.