C.C.
I remember before my second child was born, I was trying to gain some perspective on what it would be like for my older child (I'm an only child, so I had no personal experience with this). I read a book that said, imagine if one day your husband came home and told you, "Hey, honey, guess what? I'm bringing home a new wife! You'll love her! She's so cute. I'll probably be spending all of my time taking care of her, and you'll have to take care of her, too. I'm doing this for you! Aren't you excited?" And THAT's how children feel at the prospect of a new brother or sister - they don't want to share the attention of the people they love the most, and it feels like a betrayal to them. This was a revelation to me.
What I did with my oldest was to emphasize how great it is to be a "big kid." She is able to do so many things - ride a bike, climb on the jungle gym, dress herself, help in the kitchen, swim... babies can't do that stuff. They just lay there and drool, poop, and cry, poor little things. And I was also honest with her. Babies do require a lot of attention and care. That doesn't mean Mommy loves Baby more, it just means that babies need to eat all the time! I've also never told my kids that I love them both the same. That's not true - I love them for different reasons. I love my older daughter's independence, intelligence, grace, and humor. I love my younger daughter's sweetness, zany personality, curiosity, and imagination. They are totally different people, so I could never love them just the same. My younger daughter, therefore, could never be a replacement for my older daughter, and vice versa - and they know this.
One thing I would really encourage is for you to make a date with your daughter weekly, or more often if you can manage it, to do things with her while someone else watches her little brothers. (Likewise, do this with your older son.) This way she knows that you value spending time with her, and she has your undivided attention during that time. It might even be something as simple as taking her to the park before soccer practice, or going for an ice cream after school. It's important for her to know that she has a special place in your heart and in your life - it's scary for her to think that she is being displaced.