You need to make yourself very clear with each of your 3 girls. Let me explain what I mean.
Several years ago as a new high school teacher, I once made the mistake of trying to discipline a group of (challenging) students at once/in front of the others. Because these students had been doing what they wanted prior to my class, they knew exactly what to do to break down my classroom management to get what they wanted. The moment I went to take care of one situation, several others in the class would chime in to defend/excuse the student in trouble. It very quickly turned into a situation where it was the class against me...which is not a good thing when you have a roomful of teenagers. Long story short, we got through the situation, punishments were still divvied out, and we moved on. BUT, I learned a few very valuable lessons from this. First of all, you've got to separate the student from the 'herd.' Also, try to discipline with courtesy to the child in question. It's not always possible, but when it is possible, pull that child aside, explain what you observed. Give a POSITIVE statement, follow up with what you expect your child to do, then ask for her compliance in the future. This is actually really fast to do once you've trained yourself. For example, "Hey, Amy, I know that you are a really independent person, and this is very comforting to me, because I know that you will be able to take care of yourself in difficult situations as you become an adult. However, when you just spoke to me like that, I felt very disrespected by your tone of voice. I know that you can speak in a much more respectful tone, because you have shown me that you can do this. In the future, when you disagree with something I say, could we talk about it instead of making disrespectful comments? Is that something you would be willing to try?" Keep it positive, calm, and quick...
Also, you need to communicate SEPARATELY with each of your older girls about your expectations for them. I really liked Patricia's comments about your oldest daughter...she may really respond to being treated like an adult. :)
Ultimately, though, your oldest two need to understand that if they intervene, then they will have consequences, as well. Then follow through on those consequences!