Get quieter. You have inadvertently trained her to "rely on the fact" that she can safely ignore you until the third (and louder) command. Which may also mean that she has inadvertently "trained" you to give her three chances before she has to act.
Upset the routine. Go quiet on her, speaking softly, and backing it up with instant action when you don't get the response you demand. She'll get it, quickly... when it costs her something she really wants. Get a number system in place if that seems to work, 1, 2, 3 (since she likes the number three already...just have it on your own terms, (it does work well with some kids,) IF you are very consistent, and determine a consequence she doesn't want(time out, naughty place (a la SuperNanny), losing something she really treasures for a set period of time, etc.) and then stick to it, no matter how much she tests you or tries to wear you down in order to reestablish control. No drama, just implacable ol' mom, can't "work her anymore", "what am I going to do now that I can't push her buttons?"
Think about what the Dog Whisperer Cesar Milano says about being the "pack leader" to his dogs...that when a dog feels/sees you're not a competent pack leader, he/she feels a need to become the pack leader him/herself, and will take control until you wrest it back. "Calm, assertive discipline" is his watchword. Check it out. I was amazed at how appropriate much of what he says applies to young children as well, who at that age are in full-time training mode, needing to explore, test, but learn HOW to interact with others, including you and the siblings.
Verbalize that you admire and regard a girl when you
"catch one of them doing something right", so that she sees this, sees what good behavior earns, gets the idea that being obedient is better than getting attention by being bad. She gets more of you by being bad right now, and for many kids, negative behavior must of necessity pull the attention away from the good children. It works for them even if it carries negative consequences. Weird kids, those high energy ones! LOL.
So change the paradigm, be unpredictable for a time to get her attention again, and then settle into a deadly- consistent routine that takes the "control" back, putting you into the driver's seat again. Remind yourself daily, "I am the adult! She is the child!" Kids can be SO creative in getting needs met that it can bamboozle you. YOU have to be the pack leader. Calm, assertive discipline, exercise, THEN affection...remember Cesar!
Best wishes!