Hysterical three year old girl?

What I would do is make her stand in front of you whilst you come down to her level, and tell her to stop crying and calm down in a firm, controlled voice. Once she has stopped crying (this could take a few minutes depending on how hysterical she is) ask her what is wrong. Tell her firmly that she must cheer up and if she is well behaved she can have a treat (like some sweets or a toy etc). Let me know if this helps!