The daughter of a coworker of mine, who recently entered the 7th grade, is facing a similar problem: upon returning from summer break, her (now ex-)best friend mysteriously began criticizing her outfits and abandoning her for more popular girls.
Losing friends, especially for shallow and superficial reasons beyond one’s control, is an unfortunate part of life that leaves the snubbed individual of any age feeling hurt, confused, and lonely. Assuming that you’re asking as the mother of the 3rd grader in question, my advice is to encourage your daughter to communicate with you about this problem, resist the impulse to involve yourself (e.g. contacting her “friends’” parents), make yourself as available as possible to spend time with her, as the process of making new friends can be lonely, and avoid criticisms at this sensitive time, even those you may perceive as constructive. While she may not struggle with self-esteem issues yet, it is never too early to build a foundation of self-confidence that will be essential as she gets older, the stakes get higher, and this sort of thing happens again.
My advice to my coworker (which may be impractical- it’s hard to remember how difficult it is to be in K-12) was to remind her daughter that it always pays to be nice. As hurt as she may be, and as rewarding as it may feel to be mean back, learning to be nice even in these sorts of situations will enable her to make new friends quickly and easily. (One of the only positive aspects of children’s TV programs these days is the reinforcement of this message; shows like Hannah Montana valorize nice girls and make mean behavior seem as shallow, petty, and undesirable as it really is.)