I'd start by accepting you probably have no legal recourse as when an adult hands money to another adult, without signing anything to validate the money is the property of A and not B

You’re normally screwed legally

So its parents with a hope they either shame 3k out of her pockets or help you out.

Other than that, you learned a hard lesson that many of us have.

The people closest to you are sometimes thieving fuckers, and this is exactly how they react when their betrayal is uncovered.