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?
A classic source for the Upsherin is the שערי תשובה in שו"ע או"ח סי' תקלא סק"ז
המנהג בארץ ישראל לעשות שמחה בתגלחת הראשונה של קטן, שמחנכין אותו במצוה להיות לו פאות הראש.
"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.
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
the first haircut is about teaching the child about the Mitzvah of Pe'ot, since we cut the hair and leave the Pe'ot.
Why do most people do it at 3, because age 3 is associated with starting to teach children about Mitzvos (Chinuch).
then some have a custom for girls to light candles at 3.