Happy birthday to your little girl! Our daughter also turned 3 in October. :-)
I've talked to our daughter's hair dresser (that's funny, she has one but I don't) about her hair. She has a full head of hair, but there's still a lot of that baby hair and it's really fine. Anyways, she said that it can take until age 5 for kids to get their actual hair. It can even drastically change in texture and color right around that age. I don't think it's that uncommon for 3-year olds to not have much hair - unfortunately, it's much more noticeable on little girls (especially when you see the rare 2-year old with the thickest, longest hair you've ever seen).
Both my husband's sister and my mom's sister were practically bald until after age 3. And, when it did start coming in, it was so blond/white that you could barely see it. Eventually they got a full-head of hair and it didn't stay that light.
The hairdresser did recommend cutting it by now (even if there's not much to trim). She said that it will grow better and keep it healthier. So, if you haven't, you might want to take her for her first cut. :-) I highly recommend a kid's salon, they make all the difference in the world. It's the ONE time out daughter will actually sit still and enjoy herself.