First of all, congratulations!
I am going to sound like the voice of doom and gloom here, and I really don't mean to, but I am speaking purely from my experience in my household growing up. I am sure that there are other families of 5 that had completely different experiences.
I was the oldest of 3 - it is
definitely a tough number. On threads like this, I feel like I have to advocate for the oldest child. I was 5 when my brother was born, so a bit younger than your daughter but not much. Because of her age, it will be tempting to enlist her help with the younger ones. If she enjoys it and wants to, help that's fine. Just remember that she is still one of your "kids" so please don't have her help so much that she ends up resenting the younger ones. And don't ever call her the "built in babysitter". I used to hate that!
Since she is older, you'll have issues come up where she is too big/old for something that the younger kids want to do (Pooh's playful spot quickly comes to mind), so please remember to plan for her needs as well as the little ones. Of course, you all need to make sacrifices for each other, but it is easy to have the older one give in to "the babies" more often than the other way around.
Also, when you have 3, there always potential to have an "odd man out", especially since you already have a girl and a boy. The 2 vs 1 thing was very prevelent in my family.
Don't get me wrong, I wouldn't trade my brother or sister for anything in the world (most of the time

) but there are definitely issues that come with having 3 or more kids. They're easier to deal with (or avoid) if you're conscious of them going in.
Not long ago, there was a loooong thread about having 3 kids and it was very interesting reading about peoples varying experiences. I'll see if I can find it for you.