Might just mean she has relaxed muscle tone. The shope of the body is a good indicator. You'r looking for more of a "loaf of bread" profile rather than a triangular one with a prominant backbone (that's at rest - the spine is usually more visible when they move around).
Also, look at the body compared to the size of the head. Does the head look too large or too small? If it seems in proportion, then all is well.
What's she eating and how often? At 3 years old she'll be pretty much adult, but they do grow at different rates.