Upsherin is cutting a boy's hair at or about the age of three for the first time. Is there any 'Inyan (idea) of an Upsherin for girls? Why or why not?
if by "Inyan of Upsherin" you mean hair cut then see @Danny Schoemann's answer
but if by "Inyan of Upsherin" you mean the Inyan of chinuch
from this answer
then some have a custom for girls to light candles at 3.
A classic source for the Upsherin is the שערי תשובה in שו"ע או"ח סי' תקלא סק"ז
"The custom in the land of Israel is to make a joyous occasion out of the child's first haircut, when one inaugurates him into the concept of [not destroying] his [Halachic] sideburns."
Since girls do not have the Mitzva of פאות הראש, there would be no sense to making an Upsherin ceremony for them.